## Who Wants to Be a Mathematician, Qualifying Round

Go back to the Math Jam ArchiveAoPS instructor Dave Patrick will discuss the problems on Round 1 of the 2015-16 Who Wants to Be a Mathematician national contest. We will also be joined by Mike Breen and Bill Butterworth, the creators of the game. Mike is also the host of the national finals, to be held in Seattle in January 2016.

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#### Facilitator: Dave Patrick

DPatrick
2015-10-07 19:30:24

**Welcome to the 2015-16***Who Wants to Be a Mathematician*Round 1 Math Jam!
DPatrick
2015-10-07 19:30:37

I'm Dave Patrick, and I'll be leading our discussion tonight. Many of you know me from around AoPS: I've taught dozens of AoPS classes over the past 11 years, and I've written or co-written a few of our textbooks. I also once was a contestant on ABC's

I'm Dave Patrick, and I'll be leading our discussion tonight. Many of you know me from around AoPS: I've taught dozens of AoPS classes over the past 11 years, and I've written or co-written a few of our textbooks. I also once was a contestant on ABC's

*Who Wants to Be a Millionaire*back before I started working at AoPS, when Regis was still the host. (No, I didn't win the million bucks.)
TemporalFuzz
2015-10-07 19:30:54

Awww

Awww

gamjawon
2015-10-07 19:30:54

Aww

Aww

Bonami2014
2015-10-07 19:31:06

teampeeta12301
2015-10-07 19:31:11

How far did you get?

How far did you get?

DPatrick
2015-10-07 19:31:26

Yeah, I was a little sad too. But I did win $64000, which (after taxes) paid for a new car.

Yeah, I was a little sad too. But I did win $64000, which (after taxes) paid for a new car.

DPatrick
2015-10-07 19:31:38

Joining us tonight are the co-creators of WWTBAM, Mike Breen (

Joining us tonight are the co-creators of WWTBAM, Mike Breen (

**mikebreen**) and Bill Butterworth (**TPiR**).
DPatrick
2015-10-07 19:31:48

Mike taught at Alfred University and Tennessee Technological University before becoming AMS Public Awareness Officer in 2000. He and Bill began

Mike taught at Alfred University and Tennessee Technological University before becoming AMS Public Awareness Officer in 2000. He and Bill began

*Who Wants to Be a Mathematician*for the American Mathematical Society in 2001. The first national game was in 2010. Mike has been on*Jeopardy!*and*Wheel of Fortune*(if you want to know if he won lots of money on either show, note that he is still working for a living) and may be the only person ever to cut his hand on the wheel.*Who Wants to Be a Mathematician*has so far been much safer.
DPatrick
2015-10-07 19:32:19

Bill earned an undergraduate degree in mathematics from Santa Clara University and a Ph.D. from Northwestern University, and is currently an associate professor and associate chair of mathematics at DePaul University. He shares a life-long interest in game shows with colleague Mike Breen, with whom he works as the not-so-lovely assistant on the mathematics game show

Bill earned an undergraduate degree in mathematics from Santa Clara University and a Ph.D. from Northwestern University, and is currently an associate professor and associate chair of mathematics at DePaul University. He shares a life-long interest in game shows with colleague Mike Breen, with whom he works as the not-so-lovely assistant on the mathematics game show

*Who Wants to Be a Mathematician*. In addition to authoring articles and presenting talks related to game-show mathematics, Bill served as mathematics consultant to the CBS television show*The Price is Right*from 1997 to 2009. (Hence, his username.)
DPatrick
2015-10-07 19:33:13

Here's how things will work tonight. As you've probably noticed, this classroom is

Here's how things will work tonight. As you've probably noticed, this classroom is

**moderated**, meaning that students can type into the classroom, but these comments will not go directly into the room. These comments go to the instructors, who may choose to share your comments with the room.
DPatrick
2015-10-07 19:33:33

This helps keep the session organized and on track.

This helps keep the session organized and on track.

DPatrick
2015-10-07 19:34:00

There are a lot of students here! So only a fraction of you're collective well-written comments will be passed to the entire group. Please do not take it personally if your comments do not get posted, and please do not complain about it. I expect this Math Jam to be much larger than our typical class, so please be patient with me---there are quite a few of you here tonight!!

There are a lot of students here! So only a fraction of you're collective well-written comments will be passed to the entire group. Please do not take it personally if your comments do not get posted, and please do not complain about it. I expect this Math Jam to be much larger than our typical class, so please be patient with me---there are quite a few of you here tonight!!

DPatrick
2015-10-07 19:34:21

Also, we won't be going through all the math quite as thoroughly as we do in our classes -- I can't teach all the material for every problem as we go. Another difference between tonight and our regular online classes is that it is very unlikely that we'll be able to answer every single question you ask. We usually do in our classes, but we have a large number of students tonight! So, please go ahead and ask questions, but also please understand if we aren't able to answer them all!

Also, we won't be going through all the math quite as thoroughly as we do in our classes -- I can't teach all the material for every problem as we go. Another difference between tonight and our regular online classes is that it is very unlikely that we'll be able to answer every single question you ask. We usually do in our classes, but we have a large number of students tonight! So, please go ahead and ask questions, but also please understand if we aren't able to answer them all!

DPatrick
2015-10-07 19:34:34

*Who Wants to Be a Mathematician*is run by the American Mathematical Society (AMS). The AMS promotes mathematical research, fosters excellence in mathematics education, and increases the awareness of the value of mathematics to society.
DPatrick
2015-10-07 19:34:50

Round 1 of the national contest was held last month, and consisted of 10 questions, with a 15-minute time limit. So the problems are quick: you have an average of 1.5 minutes per question. (Compare that to the AMC 10/12 which has an average of 3 minutes per question, or the AIME which has an average of 12 minutes per question.)

Round 1 of the national contest was held last month, and consisted of 10 questions, with a 15-minute time limit. So the problems are quick: you have an average of 1.5 minutes per question. (Compare that to the AMC 10/12 which has an average of 3 minutes per question, or the AIME which has an average of 12 minutes per question.)

DPatrick
2015-10-07 19:35:13

We'll cover all 10 problems, but we'll take a bit longer than 15 minutes tonight, because we'll stop along the way to discuss each question. Please also remember that the purpose of this Math Jam is to work through the

We'll cover all 10 problems, but we'll take a bit longer than 15 minutes tonight, because we'll stop along the way to discuss each question. Please also remember that the purpose of this Math Jam is to work through the

**solutions**to the problems, and not to merely present the answers. "Working through the solutions" often includes discussing problem-solving tactics. So please, when a question is posted, do not simply respond with the final answer. That's not why we're here. We're going to work through the problems step-by-step.
DPatrick
2015-10-07 19:35:33

And several of the questions have interesting sidetracks, so we'll also stop and view some of the scenery along the way.

And several of the questions have interesting sidetracks, so we'll also stop and view some of the scenery along the way.

DPatrick
2015-10-07 19:35:57

If you have questions about the how the contest works, please save them for the end, where I, Mike, and/or Bill will try to answer them.

If you have questions about the how the contest works, please save them for the end, where I, Mike, and/or Bill will try to answer them.

DPatrick
2015-10-07 19:36:09

For next, let's start with the problems!

For next, let's start with the problems!

DPatrick
2015-10-07 19:36:21

Like a lot of math contests, WWTBAM starts off with a problem that most people would probably describe as "routine" -- it's just like an exercise that you would do in an algebra class.

Like a lot of math contests, WWTBAM starts off with a problem that most people would probably describe as "routine" -- it's just like an exercise that you would do in an algebra class.

DPatrick
2015-10-07 19:36:24

1. Solve for $x$: $2x^2-x=15.$

1. Solve for $x$: $2x^2-x=15.$

DPatrick
2015-10-07 19:36:39

(You'll notice that I always put the current problem under discussion at the top of the window.)

(You'll notice that I always put the current problem under discussion at the top of the window.)

stan23456
2015-10-07 19:36:59

Subtract 15 on both sides and factor

Subtract 15 on both sides and factor

checkmatetang
2015-10-07 19:36:59

Move all terms to one side then solve the quadratic.

Move all terms to one side then solve the quadratic.

Rubaiya
2015-10-07 19:36:59

bring it all to the rhs then factor

bring it all to the rhs then factor

szhang7853
2015-10-07 19:36:59

2x^2-x-15=0 then solve using quadratic formula or by factoring

2x^2-x-15=0 then solve using quadratic formula or by factoring

DPatrick
2015-10-07 19:37:07

Right, we can write this more traditionally as a quadratic equation as $2x^2-x-15 = 0.$

Right, we can write this more traditionally as a quadratic equation as $2x^2-x-15 = 0.$

trumpeter
2015-10-07 19:37:25

quadratic formula

quadratic formula

mathwhiz16
2015-10-07 19:37:25

You can use the quadratic formula or try to factor it.

You can use the quadratic formula or try to factor it.

mjlove
2015-10-07 19:37:25

factor

factor

Einsteinhead
2015-10-07 19:37:32

Then plug into the quadratic formula!

Then plug into the quadratic formula!

Bonami2014
2015-10-07 19:37:32

now use quadratic formula

now use quadratic formula

DPatrick
2015-10-07 19:37:39

At this point, there's always the quadratic formula: $$x = \frac{1 \pm \sqrt{1 - 4(2)(-15)}}{4} = \frac{1 \pm \sqrt{121}}{4}.$$

At this point, there's always the quadratic formula: $$x = \frac{1 \pm \sqrt{1 - 4(2)(-15)}}{4} = \frac{1 \pm \sqrt{121}}{4}.$$

trish
2015-10-07 19:38:04

So the answers are 3 and -5/2

So the answers are 3 and -5/2

Natsu_Dragneel
2015-10-07 19:38:04

3 or -5/2

3 or -5/2

phi_ftw1618
2015-10-07 19:38:04

That simplifies to 12/4 and -10/4

That simplifies to 12/4 and -10/4

DPatrick
2015-10-07 19:38:10

At this point we notice that $\sqrt{121} = 11$, so our solutions are $x = \dfrac{1 \pm 11}{4}.$

At this point we notice that $\sqrt{121} = 11$, so our solutions are $x = \dfrac{1 \pm 11}{4}.$

skiboy32
2015-10-07 19:38:18

3, -5/2

3, -5/2

adas1
2015-10-07 19:38:18

3 or -5/2

3 or -5/2

teampeeta12301
2015-10-07 19:38:18

3 and -5/2

3 and -5/2

DPatrick
2015-10-07 19:38:20

That gives us $\boxed{x=3}$ and $\boxed{x=-\frac52}$ as our solutions.

That gives us $\boxed{x=3}$ and $\boxed{x=-\frac52}$ as our solutions.

Einsteinhead
2015-10-07 19:38:39

Factoring is much nicer

Factoring is much nicer

stan23456
2015-10-07 19:38:39

(2x+5)(x-3)=0, so x=3,-5/2

(2x+5)(x-3)=0, so x=3,-5/2

Liopleurodon
2015-10-07 19:38:39

we could factor it as well

we could factor it as well

mihirb
2015-10-07 19:38:39

or $(2x+5)(x-3) = 0$

or $(2x+5)(x-3) = 0$

goodbear
2015-10-07 19:38:39

$(x-3)(2x+5)=0$

$(x-3)(2x+5)=0$

winnertakeover
2015-10-07 19:38:39

Factor (2x+5)(x-3)

Factor (2x+5)(x-3)

teampeeta12301
2015-10-07 19:38:39

We could have just factored

We could have just factored

DPatrick
2015-10-07 19:38:50

Certainly, we could have solved this quadratic by factoring: $$(2x^2-x-15) = (2x+5)(x-3).$$ So $2x+5 = 0$, giving $x=-\frac52$, or $x-3=0$, giving $x=3$.

Certainly, we could have solved this quadratic by factoring: $$(2x^2-x-15) = (2x+5)(x-3).$$ So $2x+5 = 0$, giving $x=-\frac52$, or $x-3=0$, giving $x=3$.

DPatrick
2015-10-07 19:39:08

Either way, as a check of our solutions, we can use Vieta's Formulas for a quadratic. What do these formulas tell us?

Either way, as a check of our solutions, we can use Vieta's Formulas for a quadratic. What do these formulas tell us?

phi_ftw1618
2015-10-07 19:39:32

Sum and product of solutions?

Sum and product of solutions?

winnertakeover
2015-10-07 19:39:32

sum and product of roots

sum and product of roots

Natsu_Dragneel
2015-10-07 19:39:32

-b/a is the sum of the roots, and c/a is the product of them

-b/a is the sum of the roots, and c/a is the product of them

mjlove
2015-10-07 19:39:32

sum and product of roots

sum and product of roots

goodbear
2015-10-07 19:39:32

the product and sum of the numbers

the product and sum of the numbers

Bonami2014
2015-10-07 19:39:32

the sum and product of the roots

the sum and product of the roots

DPatrick
2015-10-07 19:39:49

Exactly. In general, Vieta's Formulas tell us that in the quadratic $ax^2 + bx + c = 0$, the two roots must sum to $-\frac{b}{a}$ and have a product of $\frac{c}{a}$.

Exactly. In general, Vieta's Formulas tell us that in the quadratic $ax^2 + bx + c = 0$, the two roots must sum to $-\frac{b}{a}$ and have a product of $\frac{c}{a}$.

DPatrick
2015-10-07 19:40:04

So for our quadratic, the sum of the roots should be $\frac12$, and the product of the roots should be $-\frac{15}{2}$.

So for our quadratic, the sum of the roots should be $\frac12$, and the product of the roots should be $-\frac{15}{2}$.

DPatrick
2015-10-07 19:40:18

Sure enough, this holds for the two roots that we found, so we can be sure that they're correct.

Sure enough, this holds for the two roots that we found, so we can be sure that they're correct.

DPatrick
2015-10-07 19:40:34

Onwards to #2!

Onwards to #2!

DPatrick
2015-10-07 19:40:42

2. What is the ones digit of $2017^{2015}?$

2. What is the ones digit of $2017^{2015}?$

DPatrick
2015-10-07 19:40:58

What the first simplification that we want to make?

What the first simplification that we want to make?

trumpeter
2015-10-07 19:41:15

get rid of the 201

get rid of the 201

cyborg108
2015-10-07 19:41:15

7^2015

7^2015

Rubaiya
2015-10-07 19:41:15

same as $7^{2015}$

same as $7^{2015}$

gamjawon
2015-10-07 19:41:15

$7^{2015}$

$7^{2015}$

mihirb
2015-10-07 19:41:15

$7^{2015}$

$7^{2015}$

Natsu_Dragneel
2015-10-07 19:41:15

You just do 7^2015

You just do 7^2015

DarkPikachu
2015-10-07 19:41:15

7^2015 is the same

7^2015 is the same

letsgomath
2015-10-07 19:41:15

7^2015

7^2015

jxiao
2015-10-07 19:41:15

It is the same as 7^2015

It is the same as 7^2015

DivideBy0
2015-10-07 19:41:15

same as 7^2015

same as 7^2015

DPatrick
2015-10-07 19:41:20

We know that the units digit of a product (or a power, which is a special type of product) depends only on the units digits of what we're starting with.

We know that the units digit of a product (or a power, which is a special type of product) depends only on the units digits of what we're starting with.

DPatrick
2015-10-07 19:41:29

So $2017^{2015}$ has the same units digit as $7^{2015}$.

So $2017^{2015}$ has the same units digit as $7^{2015}$.

Liopleurodon
2015-10-07 19:41:49

Find a pattern!

Find a pattern!

teampeeta12301
2015-10-07 19:41:49

Look for a pattern

Look for a pattern

eveningstarandlion
2015-10-07 19:41:49

Find a pattern in the unit digit with smaller powers

Find a pattern in the unit digit with smaller powers

rt03
2015-10-07 19:41:49

Make a pattern of the units digit

Make a pattern of the units digit

atmchallenge
2015-10-07 19:41:49

Now let's find a pattern!

Now let's find a pattern!

Rubaiya
2015-10-07 19:41:49

now look for a patttern

now look for a patttern

phi_ftw1618
2015-10-07 19:41:49

Then we need to find the cycle of 7^n

Then we need to find the cycle of 7^n

DPatrick
2015-10-07 19:41:55

Good idea. We can look at small powers of 7, and see if we can figure out the pattern.

Good idea. We can look at small powers of 7, and see if we can figure out the pattern.

gamjawon
2015-10-07 19:42:19

7, 9, 3, 1 is the pattern of the increasing powers

7, 9, 3, 1 is the pattern of the increasing powers

cyborg108
2015-10-07 19:42:19

so find the pattern (7,9,3,1)

so find the pattern (7,9,3,1)

szhang7853
2015-10-07 19:42:19

The units digits of the powers of 7 cycle every 4, so it's 7 9 3 1 7 9 3 1 and on and on

The units digits of the powers of 7 cycle every 4, so it's 7 9 3 1 7 9 3 1 and on and on

TheLuckyAngelo
2015-10-07 19:42:19

the cycle is 1,7,9,3,1,...

the cycle is 1,7,9,3,1,...

vsny23
2015-10-07 19:42:19

3 b/c 7,9,3,1...

3 b/c 7,9,3,1...

jxiao
2015-10-07 19:42:19

The pattern is 7, 9, 3, 1, repeat.

The pattern is 7, 9, 3, 1, repeat.

Einsteinhead
2015-10-07 19:42:19

It repeats: 7, 9, 3, 1, 7...

It repeats: 7, 9, 3, 1, 7...

gamjawon
2015-10-07 19:42:19

7, 9, 3, 1 is the pattern

7, 9, 3, 1 is the pattern

mihirb
2015-10-07 19:42:19

$7,9,3,1,7,9,3,1$

$7,9,3,1,7,9,3,1$

Flash12
2015-10-07 19:42:19

7, 9, 3, 1, 7, 9, 3, 1.... etc

7, 9, 3, 1, 7, 9, 3, 1.... etc

TheLuckyAngelo
2015-10-07 19:42:19

The cycle repeats by four!

The cycle repeats by four!

DPatrick
2015-10-07 19:42:24

\begin{align*}

\text{units digit of } 7^1 &= 7, \\

\text{units digit of } 7^2 &= 9, \\

\text{units digit of } 7^3 &= 3, \\

\text{units digit of } 7^4 &= 1, \\

\text{units digit of } 7^5 &= 7, \\

&\vdots

\end{align*}

\begin{align*}

\text{units digit of } 7^1 &= 7, \\

\text{units digit of } 7^2 &= 9, \\

\text{units digit of } 7^3 &= 3, \\

\text{units digit of } 7^4 &= 1, \\

\text{units digit of } 7^5 &= 7, \\

&\vdots

\end{align*}

DPatrick
2015-10-07 19:42:31

We can see that the pattern repeats for every power of 4.

We can see that the pattern repeats for every power of 4.

DPatrick
2015-10-07 19:42:37

So we just need to figure out where $7^{2015}$ fits in the pattern.

So we just need to figure out where $7^{2015}$ fits in the pattern.

mjlove
2015-10-07 19:42:59

divide 2015 by 4

divide 2015 by 4

MathStudent2002
2015-10-07 19:42:59

2015 divided by 4 leaves a remainder of 3

2015 divided by 4 leaves a remainder of 3

Natsu_Dragneel
2015-10-07 19:42:59

So you divide 2015 by 4, giving a remainder of 3.

So you divide 2015 by 4, giving a remainder of 3.

Destructio
2015-10-07 19:42:59

2015mod4

2015mod4

WL0410
2015-10-07 19:42:59

2015 is 3 mod 4

2015 is 3 mod 4

Ramanan369
2015-10-07 19:42:59

2015 mod 4

2015 mod 4

mathwhiz16
2015-10-07 19:42:59

It is 3 mod 4

It is 3 mod 4

Flash12
2015-10-07 19:42:59

2015 mod 4 = 3, so the ones digit is 3

2015 mod 4 = 3, so the ones digit is 3

DPatrick
2015-10-07 19:43:20

Right. To be a little more formal, we can write $2015 = 4 \cdot 503 + 3$.

Right. To be a little more formal, we can write $2015 = 4 \cdot 503 + 3$.

DPatrick
2015-10-07 19:43:28

So $7^{2015} = 7^{4 \cdot 503 + 3} = (7^4)^{503} \cdot 7^3$ has units digit $1 \cdot 3 = \boxed{3}$.

So $7^{2015} = 7^{4 \cdot 503 + 3} = (7^4)^{503} \cdot 7^3$ has units digit $1 \cdot 3 = \boxed{3}$.

mihirb
2015-10-07 19:43:45

or you could use euler's totient function but this way is sometimes faster

or you could use euler's totient function but this way is sometimes faster

DPatrick
2015-10-07 19:43:56

Indeed: a more general way to approach problems of this type is to use Euler's Theorem, which states that if $a$ and $n$ are relatively prime positive integers, then $$a^{\phi(n)} \equiv 1 \pmod{n},$$

where $\phi(n)$ is the

Indeed: a more general way to approach problems of this type is to use Euler's Theorem, which states that if $a$ and $n$ are relatively prime positive integers, then $$a^{\phi(n)} \equiv 1 \pmod{n},$$

where $\phi(n)$ is the

*Euler totient function*: the number of positive integers less than $n$ that are relatively prime to $n$.
DPatrick
2015-10-07 19:44:12

(Remember that name Euler. We'll see it again a little later tonight!)

(Remember that name Euler. We'll see it again a little later tonight!)

DPatrick
2015-10-07 19:44:29

In our problem, $a=7$, $n=10$, and $\phi(n) = 4$ (since 1, 3, 7, and 9 are the numbers less than 10 that are relatively prime to 10). So we have $7^4 \equiv 1 \pmod{10}$, which tells us exactly that the units digit of $7^4$ is 1.

In our problem, $a=7$, $n=10$, and $\phi(n) = 4$ (since 1, 3, 7, and 9 are the numbers less than 10 that are relatively prime to 10). So we have $7^4 \equiv 1 \pmod{10}$, which tells us exactly that the units digit of $7^4$ is 1.

DPatrick
2015-10-07 19:44:53

From there, we can proceed as we did above: $7^{2015} = 7^{4 \cdot 503 + 3} = (7^4)^{503} \cdot 7^3$ has units digit $1 \cdot 3 = \boxed{3}$.

From there, we can proceed as we did above: $7^{2015} = 7^{4 \cdot 503 + 3} = (7^4)^{503} \cdot 7^3$ has units digit $1 \cdot 3 = \boxed{3}$.

DPatrick
2015-10-07 19:45:25

A special case of Euler's Theorem is

A special case of Euler's Theorem is

*Fermat's Little Theorem*, which is the case where $n$ is prime (and we relabel it $p$).
DPatrick
2015-10-07 19:45:29

What's $\phi(p)$ if $p$ is prime?

What's $\phi(p)$ if $p$ is prime?

High
2015-10-07 19:45:50

p-1

p-1

phi_ftw1618
2015-10-07 19:45:50

p-1?

p-1?

mjlove
2015-10-07 19:45:50

p-1

p-1

nosaj
2015-10-07 19:45:50

$p-1$

$p-1$

ImpossibleCube
2015-10-07 19:45:50

p-1

p-1

goodbear
2015-10-07 19:45:50

p-1

p-1

DPatrick
2015-10-07 19:45:55

Right. Every positive integer less than $p$ is relatively prime to $p$ if $p$ is prime. So $\phi(p) = p-1$.

Right. Every positive integer less than $p$ is relatively prime to $p$ if $p$ is prime. So $\phi(p) = p-1$.

DPatrick
2015-10-07 19:46:07

And then Fermat's Little Theorem says that, provided $a$ is not a multiple of $p$, \[a^{p-1} \equiv 1 \pmod{p}.\]

And then Fermat's Little Theorem says that, provided $a$ is not a multiple of $p$, \[a^{p-1} \equiv 1 \pmod{p}.\]

DPatrick
2015-10-07 19:46:34

This, of course, is more advanced math than you'd need to solve this problem -- but I said we'd be stopping and looking at some scenery along the way.

This, of course, is more advanced math than you'd need to solve this problem -- but I said we'd be stopping and looking at some scenery along the way.

DPatrick
2015-10-07 19:46:50

On to #3!

On to #3!

mikebreen
2015-10-07 19:46:52

Love the scenic route!

Love the scenic route!

DPatrick
2015-10-07 19:46:57

3. How many solutions are there in $[0,2\pi]$ to the equation $\sin x = \cos x?$

3. How many solutions are there in $[0,2\pi]$ to the equation $\sin x = \cos x?$

DPatrick
2015-10-07 19:47:09

How would/did you solve this problem?

How would/did you solve this problem?

rt03
2015-10-07 19:47:42

Maybe make a graph?

Maybe make a graph?

mikhailgromov
2015-10-07 19:47:42

Draw a graph

Draw a graph

nosaj
2015-10-07 19:47:42

I solved this problem by graphing and finding intersections.

I solved this problem by graphing and finding intersections.

DarkPikachu
2015-10-07 19:47:42

graph it

graph it

goldentail141
2015-10-07 19:47:42

Graph sketching

Graph sketching

High
2015-10-07 19:47:42

graph the lines

graph the lines

DPatrick
2015-10-07 19:47:49

Sure, one way is to sketch the graphs $y=\sin x$ and $y=\cos x$:

Sure, one way is to sketch the graphs $y=\sin x$ and $y=\cos x$:

DPatrick
2015-10-07 19:47:54

DPatrick
2015-10-07 19:48:02

Quick quiz: which is which?

Quick quiz: which is which?

szhang7853
2015-10-07 19:48:24

red is cosx

red is cosx

atmchallenge
2015-10-07 19:48:24

red=$\cos x$

red=$\cos x$

jxiao
2015-10-07 19:48:24

The blue is sin

The blue is sin

atmchallenge
2015-10-07 19:48:24

blue= $\sin x$

blue= $\sin x$

DPatrick
2015-10-07 19:48:29

That's right: cosine is in red and sine is in blue.

That's right: cosine is in red and sine is in blue.

DPatrick
2015-10-07 19:48:39

If we're reasonably confident that these graphs are sufficiently accurate, then there are clearly 2 intersection points, which means that there are $\boxed{2}$ values of $x$ at which $\sin x = \cos x$.

If we're reasonably confident that these graphs are sufficiently accurate, then there are clearly 2 intersection points, which means that there are $\boxed{2}$ values of $x$ at which $\sin x = \cos x$.

trumpeter
2015-10-07 19:49:01

$\tan x=1$

$\tan x=1$

jxiao
2015-10-07 19:49:01

Look when tan x = 1

Look when tan x = 1

mjlove
2015-10-07 19:49:01

divide cosx, find tanx=1

divide cosx, find tanx=1

EulerMacaroni
2015-10-07 19:49:01

Think $\tan{x}=1$

Think $\tan{x}=1$

chezbgone
2015-10-07 19:49:01

Divide by $\cos x$ to get $\tan x$

Divide by $\cos x$ to get $\tan x$

EulerMacaroni
2015-10-07 19:49:01

$\tan{x}=1$, then you look at $x=\frac{\pi}{4}$ and $x=\frac{5\pi}{4}$

$\tan{x}=1$, then you look at $x=\frac{\pi}{4}$ and $x=\frac{5\pi}{4}$

DPatrick
2015-10-07 19:49:07

Right, another option is to divide both sides of the original equation by $\cos x$, giving $\tan x = 1$.

Right, another option is to divide both sides of the original equation by $\cos x$, giving $\tan x = 1$.

DPatrick
2015-10-07 19:49:18

Now we use our knowledge of the tangent function to note that $x = \frac{\pi}{4}$ and $x = \frac{5\pi}{4}$ are the solutions for $0 \le x \le 2\pi$, so again we have $\boxed{2}$ solutions (and in fact we know what those solutions are!).

Now we use our knowledge of the tangent function to note that $x = \frac{\pi}{4}$ and $x = \frac{5\pi}{4}$ are the solutions for $0 \le x \le 2\pi$, so again we have $\boxed{2}$ solutions (and in fact we know what those solutions are!).

DPatrick
2015-10-07 19:49:32

However: what do we have to be careful about whenever we divide an equation like this?

However: what do we have to be careful about whenever we divide an equation like this?

phi_ftw1618
2015-10-07 19:49:51

Division by 0?

Division by 0?

EulerMacaroni
2015-10-07 19:49:51

dividing by zero

dividing by zero

mathwhiz16
2015-10-07 19:49:51

cos(x)=0

cos(x)=0

Knin2820
2015-10-07 19:49:51

Division by 0

Division by 0

szhang7853
2015-10-07 19:49:51

when cosx=0

when cosx=0

WL0410
2015-10-07 19:49:51

Dividing by 0

Dividing by 0

stronto
2015-10-07 19:49:51

it doesnt equal 0

it doesnt equal 0

guluguluga
2015-10-07 19:49:51

can't divide by 0

can't divide by 0

DPatrick
2015-10-07 19:49:56

We have to be careful not to divide by 0. Otherwise we might lose a solution.

We have to be careful not to divide by 0. Otherwise we might lose a solution.

DPatrick
2015-10-07 19:50:02

For example, if there were a value of $x$ for which $\cos x = 0$ that solved the equation, then we'd lose that solution when we divide by $\cos x$.

For example, if there were a value of $x$ for which $\cos x = 0$ that solved the equation, then we'd lose that solution when we divide by $\cos x$.

Liopleurodon
2015-10-07 19:50:14

but it's impossible!

but it's impossible!

EulerMacaroni
2015-10-07 19:50:17

doesn't matter since $\sin$ and $\cos$ are never zero at the same time

doesn't matter since $\sin$ and $\cos$ are never zero at the same time

DPatrick
2015-10-07 19:50:28

Right you all. Because $\sin^2x + \cos^2x = 1$ always, we can't have them both be 0 at the same time.

Right you all. Because $\sin^2x + \cos^2x = 1$ always, we can't have them both be 0 at the same time.

DPatrick
2015-10-07 19:50:53

On to #4!

On to #4!

DPatrick
2015-10-07 19:50:57

One of the unique and (in my opinion) fun aspects of WWTBAM is that there is always a question about the history of mathematics:

One of the unique and (in my opinion) fun aspects of WWTBAM is that there is always a question about the history of mathematics:

DPatrick
2015-10-07 19:51:00

4. George Boole, who developed the logic upon which computers operate, was born in which of the following countries?

(a) Austria (b) England (c) France (d) Germany

4. George Boole, who developed the logic upon which computers operate, was born in which of the following countries?

(a) Austria (b) England (c) France (d) Germany

DPatrick
2015-10-07 19:51:12

If you think it'll help, here's a picture of Mr. Boole:

If you think it'll help, here's a picture of Mr. Boole:

DPatrick
2015-10-07 19:51:17

Liopleurodon
2015-10-07 19:51:40

England!

England!

szhang7853
2015-10-07 19:51:40

his name sounds englishh so b

his name sounds englishh so b

EulerMacaroni
2015-10-07 19:51:40

His name is George so it's clearly England

His name is George so it's clearly England

Liopleurodon
2015-10-07 19:51:40

His name sounds english lol

His name sounds english lol

szhang7853
2015-10-07 19:51:40

george is an english name

george is an english name

eveningstarandlion
2015-10-07 19:51:45

England actually

England actually

joeyusa2013
2015-10-07 19:51:45

b. England

b. England

DPatrick
2015-10-07 19:51:51

George Boole was born on November 2, 1815, in Lincoln, Lincolnshire,

George Boole was born on November 2, 1815, in Lincoln, Lincolnshire,

**England**. So the answer is $\boxed{(\text{b})}$. (However, Boole spent much of his adult life in Ireland, where he was a mathematics professor.)
mikebreen
2015-10-07 19:52:01

Boole making all this possible.

Boole making all this possible.

DPatrick
2015-10-07 19:52:16

Indeed! Although Boole did research in a variety of mathematical topics, he is best known for developing the foundations of what we today call

Indeed! Although Boole did research in a variety of mathematical topics, he is best known for developing the foundations of what we today call

**Boolean algebra**, which is a mathematical description of logical operations, and the foundation of much computer programming.
DPatrick
2015-10-07 19:52:26

In fact, many computer programming languages using the term

In fact, many computer programming languages using the term

**boolean**to refer to a variable that can take the values "true" or "false". These variables can then be manipulated using Boolean algebra operations, as in this snippet from the Python programming language:
DPatrick
2015-10-07 19:52:30

DPatrick
2015-10-07 19:52:40

Go ahead and click "Run"!

Go ahead and click "Run"!

DPatrick
2015-10-07 19:53:13

By the way, my source for the photo and the biographical information is the page http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Boole.html, which is part of the great MacTutor History of Mathematics archive at the University of St. Andrews in Scotland. The main page is http://www-groups.dcs.st-and.ac.uk/~history. They have biographies of hundreds of mathematicians, from Asger Aaboe to Eustachy Zylinski. (No, I didn't make those names up.)

By the way, my source for the photo and the biographical information is the page http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Boole.html, which is part of the great MacTutor History of Mathematics archive at the University of St. Andrews in Scotland. The main page is http://www-groups.dcs.st-and.ac.uk/~history. They have biographies of hundreds of mathematicians, from Asger Aaboe to Eustachy Zylinski. (No, I didn't make those names up.)

DPatrick
2015-10-07 19:53:47

And a shameless plug: if you'd even like to learn the Python programming language, a snippet of which is above, AoPS has Python classes!

And a shameless plug: if you'd even like to learn the Python programming language, a snippet of which is above, AoPS has Python classes!

DPatrick
2015-10-07 19:54:03

Anyway, on to problem #5:

Anyway, on to problem #5:

DPatrick
2015-10-07 19:54:06

5. How many subsets (including the set itself) of a four-element set have at least two elements?

5. How many subsets (including the set itself) of a four-element set have at least two elements?

nosaj
2015-10-07 19:54:36

We can do casework on the number of elements in the subset!

We can do casework on the number of elements in the subset!

mihirb
2015-10-07 19:54:36

Do case work on the number of elements

Do case work on the number of elements

rt03
2015-10-07 19:54:36

Divide into cases based on the number of elements in the subset

Divide into cases based on the number of elements in the subset

DPatrick
2015-10-07 19:54:46

One way to approach this problem is to use casework. Specifically, if a subset of a 4-element set has at least two elements, then it has either 2, 3, or 4 elements.

One way to approach this problem is to use casework. Specifically, if a subset of a 4-element set has at least two elements, then it has either 2, 3, or 4 elements.

phi_ftw1618
2015-10-07 19:55:07

Isn't it just $4C2+4C3+4C4$ = $6+4+1$=11?

Isn't it just $4C2+4C3+4C4$ = $6+4+1$=11?

gamjawon
2015-10-07 19:55:07

4 choose 2 + 4 choose 3 + 4 choose 4

4 choose 2 + 4 choose 3 + 4 choose 4

mihirb
2015-10-07 19:55:13

there are ${4 \choose 2}$ 2 element ones ${4 \choose 3}$ 3 element ones and ${4 \choose 4}$ 4 element ones

there are ${4 \choose 2}$ 2 element ones ${4 \choose 3}$ 3 element ones and ${4 \choose 4}$ 4 element ones

DPatrick
2015-10-07 19:55:17

Exactly.

Exactly.

vsny23
2015-10-07 19:55:21

11 b/c 4C2 plus 4C3 plus 4C4

11 b/c 4C2 plus 4C3 plus 4C4

adas1
2015-10-07 19:55:21

4C2 + 4C3 + 4C4 = 6+4+1=11

4C2 + 4C3 + 4C4 = 6+4+1=11

DPatrick
2015-10-07 19:55:28

To count 2-element subsets, we have to choose two of the four elements to form the subset. One way to express this is as the combination $\binom42$, which is 6.

To count 2-element subsets, we have to choose two of the four elements to form the subset. One way to express this is as the combination $\binom42$, which is 6.

DPatrick
2015-10-07 19:55:38

Another way to think of it is that there are 4 choices for the first element of the subset, and then 3 choices for the second element of the subset. So that's $4 \cdot 3 = 12$ ways to choose two elements.

Another way to think of it is that there are 4 choices for the first element of the subset, and then 3 choices for the second element of the subset. So that's $4 \cdot 3 = 12$ ways to choose two elements.

DPatrick
2015-10-07 19:55:44

But subsets don't care about order, so this method of choosing counts each subset twice: once for either order of choosing the two elements. Thus, when counting 12, we've double-counted, and the actual number of subsets is $12/2 = 6$.

But subsets don't care about order, so this method of choosing counts each subset twice: once for either order of choosing the two elements. Thus, when counting 12, we've double-counted, and the actual number of subsets is $12/2 = 6$.

DPatrick
2015-10-07 19:56:04

Similarly, we can count $\binom43 = 4$ ways to choose 3 elements out of 4. (But it's easier to just think of which element we decide to leave out: we have 4 choices for the missing element.)

Similarly, we can count $\binom43 = 4$ ways to choose 3 elements out of 4. (But it's easier to just think of which element we decide to leave out: we have 4 choices for the missing element.)

DPatrick
2015-10-07 19:56:18

And just 1 four-element subset: the set itself.

And just 1 four-element subset: the set itself.

DPatrick
2015-10-07 19:56:27

So our final answer is that there are $6 + 4 + 1 = \boxed{11}$ subsets with 2 or more elements.

So our final answer is that there are $6 + 4 + 1 = \boxed{11}$ subsets with 2 or more elements.

Einsteinhead
2015-10-07 19:56:41

Use complementary counting!

Use complementary counting!

atmchallenge
2015-10-07 19:56:41

Complementary counting!

Complementary counting!

mikhailgromov
2015-10-07 19:56:41

Complementary counting

Complementary counting

KevTu
2015-10-07 19:56:41

Complementary counting

Complementary counting

WL0410
2015-10-07 19:56:41

Complementary counting

Complementary counting

axue
2015-10-07 19:56:41

or complementary counting if casework is too complicated

or complementary counting if casework is too complicated

Rubaiya
2015-10-07 19:56:41

complementary

complementary

Knin2820
2015-10-07 19:56:41

Or complementary counting, no?

Or complementary counting, no?

DPatrick
2015-10-07 19:56:53

Right: another approach is that we could count all the subsets, and subtract those with fewer than 2 elements. (This is sometimes called

Right: another approach is that we could count all the subsets, and subtract those with fewer than 2 elements. (This is sometimes called

**complementary counting**: we're counting what we don't want.)
Pimaster314
2015-10-07 19:57:30

2^n = number of subsets

2^n = number of subsets

EulerMacaroni
2015-10-07 19:57:30

or just $16-4-1=11$ by complementary counting

or just $16-4-1=11$ by complementary counting

jxiao
2015-10-07 19:57:30

You can also have 2^4 = 16 sets in total, minus 1 empty set and 4 one-element sets to get 11.

You can also have 2^4 = 16 sets in total, minus 1 empty set and 4 one-element sets to get 11.

phi_ftw1618
2015-10-07 19:57:30

So that's 16 subsets - $4C0+4C1$ = 16-5 = 11

So that's 16 subsets - $4C0+4C1$ = 16-5 = 11

phi_ftw1618
2015-10-07 19:57:30

Oops forgot the parenthesis

Oops forgot the parenthesis

DPatrick
2015-10-07 19:57:45

Right. To start, there are $2^4 = 16$ subsets altogether. Think about how you would construct a subset: for each of the 4 elements, you have to decide whether it's in the subset or not. So you have 2 choices for each of 4 elements, and these choices are independent of each other, giving $2^4 = 16$ possibilities.

Right. To start, there are $2^4 = 16$ subsets altogether. Think about how you would construct a subset: for each of the 4 elements, you have to decide whether it's in the subset or not. So you have 2 choices for each of 4 elements, and these choices are independent of each other, giving $2^4 = 16$ possibilities.

DPatrick
2015-10-07 19:58:03

And then we count how many of these 16 subsets have fewer than 2 elements.

And then we count how many of these 16 subsets have fewer than 2 elements.

DPatrick
2015-10-07 19:58:10

Well, there's the empty set $\emptyset$. That's 1.

Well, there's the empty set $\emptyset$. That's 1.

DPatrick
2015-10-07 19:58:15

There's also the 4 subsets each consisting of a single element.

There's also the 4 subsets each consisting of a single element.

DPatrick
2015-10-07 19:58:20

So that's $1+4 = 5$ subsets we have to exclude, leaving us a total of $16 - 5 = \boxed{11}$ desired subsets.

So that's $1+4 = 5$ subsets we have to exclude, leaving us a total of $16 - 5 = \boxed{11}$ desired subsets.

DPatrick
2015-10-07 19:58:34

Counting problems can often be solved in multiple ways, and doing them in more than one way is a good check of your answer!

Counting problems can often be solved in multiple ways, and doing them in more than one way is a good check of your answer!

DPatrick
2015-10-07 19:58:52

Halfway home! On to #6:

Halfway home! On to #6:

DPatrick
2015-10-07 19:58:56

6. What is the area of the quadrilateral formed by gluing together a 3-4-5 right triangle and a 5-12-13 right triangle along their common side of length 5?

6. What is the area of the quadrilateral formed by gluing together a 3-4-5 right triangle and a 5-12-13 right triangle along their common side of length 5?

mjlove
2015-10-07 19:59:19

draw a diagram

draw a diagram

Pimaster314
2015-10-07 19:59:19

picture

picture

DPatrick
2015-10-07 19:59:40

This is a bit of a "trick" question in that it's much easier than it might first appear. Most of the difficulty of this problem is getting the accurate picture to be sure you're not missing anything.

This is a bit of a "trick" question in that it's much easier than it might first appear. Most of the difficulty of this problem is getting the accurate picture to be sure you're not missing anything.

DPatrick
2015-10-07 19:59:49

The side of length 5 is the hypotenuse of the smaller triangle, and the smaller leg of the larger triangle. So they glue together like this:

The side of length 5 is the hypotenuse of the smaller triangle, and the smaller leg of the larger triangle. So they glue together like this:

DPatrick
2015-10-07 19:59:53

phi_ftw1618
2015-10-07 20:00:14

Just add the triangle areas?

Just add the triangle areas?

trumpeter
2015-10-07 20:00:14

treat the triangles as separate and add the areas

treat the triangles as separate and add the areas

Harry0531
2015-10-07 20:00:14

The are right triangles!

The are right triangles!

mathwhiz16
2015-10-07 20:00:14

This is quite simple: just find the area of the two triangles separately!

This is quite simple: just find the area of the two triangles separately!

letsgomath
2015-10-07 20:00:14

find the ares of the to triangles and just add them together

find the ares of the to triangles and just add them together

ohmcfifth
2015-10-07 20:00:14

Find the areas separately and add them together

Find the areas separately and add them together

Liopleurodon
2015-10-07 20:00:14

it's just the area of the two triangles glued together!

it's just the area of the two triangles glued together!

matl26
2015-10-07 20:00:14

Just add both areas

Just add both areas

szhang7853
2015-10-07 20:00:14

find the areas of the two triangles and add them together

find the areas of the two triangles and add them together

DPatrick
2015-10-07 20:00:33

Just to be careful, we note that the entire region is a quadrilateral with sides 3, 4, 12, and 13, as required in the problem statement.

Just to be careful, we note that the entire region is a quadrilateral with sides 3, 4, 12, and 13, as required in the problem statement.

DPatrick
2015-10-07 20:00:45

So its area is just the sum of the two triangles' areas.

So its area is just the sum of the two triangles' areas.

john456852
2015-10-07 20:01:04

its 36 because (3*4/2) + (5*12/2)

its 36 because (3*4/2) + (5*12/2)

phi_ftw1618
2015-10-07 20:01:04

Then we get $\frac{3(4)}{2}+\frac{5(12)}{2}$ which is equal to $6+30$= $36$

Then we get $\frac{3(4)}{2}+\frac{5(12)}{2}$ which is equal to $6+30$= $36$

eveningstarandlion
2015-10-07 20:01:04

30+6=36

30+6=36

Wakashubi12
2015-10-07 20:01:04

36

36

int_user
2015-10-07 20:01:04

=36

=36

mihirb
2015-10-07 20:01:04

The area of the 3-4-5 is 6 and 5-12-13 30. 30+6= 36

The area of the 3-4-5 is 6 and 5-12-13 30. 30+6= 36

goodbear
2015-10-07 20:01:04

6+30=36

6+30=36

DPatrick
2015-10-07 20:01:13

The small triangle has area $\frac12 \cdot 3 \cdot 4 = 6$, and the large triangle has area $\frac12 \cdot 5 \cdot 12 = 30$.

The small triangle has area $\frac12 \cdot 3 \cdot 4 = 6$, and the large triangle has area $\frac12 \cdot 5 \cdot 12 = 30$.

DPatrick
2015-10-07 20:01:17

Thus the area of the quadrilateral is $6 + 30 = \boxed{36}$.

Thus the area of the quadrilateral is $6 + 30 = \boxed{36}$.

mikebreen
2015-10-07 20:01:33

Love watching people solve these so quickly (and so well).

Love watching people solve these so quickly (and so well).

DPatrick
2015-10-07 20:01:44

You might wonder if different gluings might lead to different areas. We could certainly flip the larger triangle:

You might wonder if different gluings might lead to different areas. We could certainly flip the larger triangle:

DPatrick
2015-10-07 20:01:48

DPatrick
2015-10-07 20:01:58

This is a different quadrilateral, but it still has the same area, given by the sum of the two triangles' areas.

This is a different quadrilateral, but it still has the same area, given by the sum of the two triangles' areas.

EulerMacaroni
2015-10-07 20:02:11

non-intersecting area is additive, you don't even need to draw a diagram

non-intersecting area is additive, you don't even need to draw a diagram

mathwhiz16
2015-10-07 20:02:14

Different gluings will not lead to different areas unless one triangle overlaps another.

Different gluings will not lead to different areas unless one triangle overlaps another.

DPatrick
2015-10-07 20:02:27

Right, we might worry a little about an overlapping gluing that still gives a quadrilateral.

Right, we might worry a little about an overlapping gluing that still gives a quadrilateral.

DPatrick
2015-10-07 20:02:33

DPatrick
2015-10-07 20:02:43

But the resulting shape is not a quadrilateral, so this is no good.

But the resulting shape is not a quadrilateral, so this is no good.

DPatrick
2015-10-07 20:02:48

Flipping the large triangle the other way also completely covers the smaller triangle by the larger one.

Flipping the large triangle the other way also completely covers the smaller triangle by the larger one.

DPatrick
2015-10-07 20:02:52

DPatrick
2015-10-07 20:03:18

OK, on to #7:

OK, on to #7:

DPatrick
2015-10-07 20:03:25

7. How many real solutions are there to the equation $\ln(x^2+x) = \ln(x^2) + \ln(x)?$

7. How many real solutions are there to the equation $\ln(x^2+x) = \ln(x^2) + \ln(x)?$

DPatrick
2015-10-07 20:03:43

First to note some slightly more advanced math than what we've seen so far: what is $\ln$?

First to note some slightly more advanced math than what we've seen so far: what is $\ln$?

Rubaiya
2015-10-07 20:04:04

natural log

natural log

ninjataco
2015-10-07 20:04:04

log base e

log base e

azmath333
2015-10-07 20:04:04

natural log

natural log

stronto
2015-10-07 20:04:04

natural log or log based e

natural log or log based e

bharatputra
2015-10-07 20:04:04

log base e

log base e

winnertakeover
2015-10-07 20:04:04

Does IN mean base e?

Does IN mean base e?

nosaj
2015-10-07 20:04:04

It's a base $e$ logarithm.

It's a base $e$ logarithm.

vsny23
2015-10-07 20:04:04

log with base e

log with base e

jxiao
2015-10-07 20:04:04

Log base e / natural log

Log base e / natural log

Harry0531
2015-10-07 20:04:04

The inverse of $e^x$

The inverse of $e^x$

ninjataco
2015-10-07 20:04:04

inverse of e^x

inverse of e^x

DPatrick
2015-10-07 20:04:12

$\ln$ is the

$\ln$ is the

**natural logarithm**, or the logarithm to the base $e$. ($e \approx 2.718281828\ldots$ is a very special irrational number...more about $e$ later.)
DPatrick
2015-10-07 20:04:23

So this is a problem about logarithms. What can we do?

So this is a problem about logarithms. What can we do?

nosaj
2015-10-07 20:04:55

Guys the domain of logs is positive numbers!!

Guys the domain of logs is positive numbers!!

EulerMacaroni
2015-10-07 20:04:55

$\ln(x^2)+\ln(x)=\ln(x^3)$

$\ln(x^2)+\ln(x)=\ln(x^3)$

bguo
2015-10-07 20:04:55

log addition property

log addition property

rt03
2015-10-07 20:04:55

Use logarithm addition rules

Use logarithm addition rules

Rubaiya
2015-10-07 20:04:55

or ln x^3

or ln x^3

ohmcfifth
2015-10-07 20:04:55

Use the sum of logs = product rule

Use the sum of logs = product rule

atmchallenge
2015-10-07 20:04:55

Logarithm rules. $\ln (x^2)+\ln (x)=\ln (x^3)$

Logarithm rules. $\ln (x^2)+\ln (x)=\ln (x^3)$

azmath333
2015-10-07 20:04:55

turn the RHS into $\ln(x^3)$ using log properties?

turn the RHS into $\ln(x^3)$ using log properties?

DPatrick
2015-10-07 20:05:14

I'd first note what

I'd first note what

**nosaj**noted: since logarithms are only defined for positive numbers, we must have $x > 0$. (This automatically makes $x^2 > 0$ and $x^2+x>0$.)
DPatrick
2015-10-07 20:05:34

Now we can use the identity $\ln(a) + \ln(b) = \ln(ab)$.

Now we can use the identity $\ln(a) + \ln(b) = \ln(ab)$.

DPatrick
2015-10-07 20:05:44

So our equation becomes $\ln(x^2+x) = \ln(x^2 \cdot x) = \ln(x^3)$.

So our equation becomes $\ln(x^2+x) = \ln(x^2 \cdot x) = \ln(x^3)$.

DPatrick
2015-10-07 20:05:51

How does this help?

How does this help?

zsp
2015-10-07 20:06:22

Take away the $ln$

Take away the $ln$

mjlove
2015-10-07 20:06:22

raise both sides to the e power

raise both sides to the e power

Rubaiya
2015-10-07 20:06:22

x^2 + x = x^3

x^2 + x = x^3

jxiao
2015-10-07 20:06:22

Now we know x^2+x=x^3.

Now we know x^2+x=x^3.

trumpeter
2015-10-07 20:06:22

$\ln$ is injective

$\ln$ is injective

WL0410
2015-10-07 20:06:22

$x^3=x^2+x$

$x^3=x^2+x$

szhang7853
2015-10-07 20:06:22

if you take both sides to the power of e, we can create an equation with no logarithms

if you take both sides to the power of e, we can create an equation with no logarithms

mikhailgromov
2015-10-07 20:06:22

Raise to power of e to get rid of logs

Raise to power of e to get rid of logs

ninjataco
2015-10-07 20:06:22

x^2 + x = x^3

x^2 + x = x^3

dtxiong
2015-10-07 20:06:22

we can set what's in the logs equal to each other

we can set what's in the logs equal to each other

mathwhiz16
2015-10-07 20:06:22

We can now change it into the equation $x^2+x=x^3$

We can now change it into the equation $x^2+x=x^3$

eveningstarandlion
2015-10-07 20:06:22

x^2+x=x^3

x^2+x=x^3

azmath333
2015-10-07 20:06:22

raise both sides to the $e$ power

raise both sides to the $e$ power

Harry0531
2015-10-07 20:06:22

That means we can $e^x$ both sides. We get $x^3=x^2+x$

That means we can $e^x$ both sides. We get $x^3=x^2+x$

DPatrick
2015-10-07 20:06:29

Right. Logarithms are 1-to-1 functions: this means that $\ln(p) = \ln(q)$ if and only if $p = q$.

Right. Logarithms are 1-to-1 functions: this means that $\ln(p) = \ln(q)$ if and only if $p = q$.

DPatrick
2015-10-07 20:06:35

So we conclude that $x^2 + x = x^3$.

So we conclude that $x^2 + x = x^3$.

DPatrick
2015-10-07 20:06:42

How do we count the solutions to this?

How do we count the solutions to this?

int_user
2015-10-07 20:07:02

now you can divide by x

now you can divide by x

mathwhiz16
2015-10-07 20:07:02

Now we can divide by x

Now we can divide by x

xayy
2015-10-07 20:07:02

divide by x

divide by x

szhang7853
2015-10-07 20:07:02

divide by x to create quadratic

divide by x to create quadratic

zsp
2015-10-07 20:07:02

Divide by x

Divide by x

ninjataco
2015-10-07 20:07:02

divide by x since x>0 and then solve the quadratic

divide by x since x>0 and then solve the quadratic

Rubaiya
2015-10-07 20:07:02

x(x^2 -x - 1) = 0

x(x^2 -x - 1) = 0

DPatrick
2015-10-07 20:07:15

Right. Clearly $x=0$ is a solution to our cubic equation. But we're discarding that solution, since $\ln(0)$ is undefined.

Right. Clearly $x=0$ is a solution to our cubic equation. But we're discarding that solution, since $\ln(0)$ is undefined.

DPatrick
2015-10-07 20:07:26

So we can divide by $x$ and we have $x + 1 = x^2$. This is the quadratic equation $x^2 - x - 1 = 0$.

So we can divide by $x$ and we have $x + 1 = x^2$. This is the quadratic equation $x^2 - x - 1 = 0$.

DPatrick
2015-10-07 20:07:38

Thus by the quadratic formula, $x = \dfrac{1 \pm \sqrt5}{2}$. So 2 solutions, right?

Thus by the quadratic formula, $x = \dfrac{1 \pm \sqrt5}{2}$. So 2 solutions, right?

ninjataco
2015-10-07 20:08:03

no, x>0

no, x>0

xayy
2015-10-07 20:08:03

no 1-sqrt5 is negative

no 1-sqrt5 is negative

goodbear
2015-10-07 20:08:03

positive only

positive only

dtxiong
2015-10-07 20:08:03

no, 1-sqrt5 is negative

no, 1-sqrt5 is negative

mjlove
2015-10-07 20:08:03

no, x cann't be negative

no, x cann't be negative

szhang7853
2015-10-07 20:08:03

1-sqrt5/2 is negative not in domain

1-sqrt5/2 is negative not in domain

mikhailgromov
2015-10-07 20:08:03

No. X>0.

No. X>0.

john456852
2015-10-07 20:08:03

no!!

no!!

gxah
2015-10-07 20:08:03

one is negative

one is negative

WL0410
2015-10-07 20:08:03

No, discard the negative one

No, discard the negative one

axue
2015-10-07 20:08:03

x cannot be negative

x cannot be negative

john456852
2015-10-07 20:08:03

its one because it can only be positive!

its one because it can only be positive!

DPatrick
2015-10-07 20:08:08

No! Remember we must have $x>0$ for the original logarithm equation to make sense!

No! Remember we must have $x>0$ for the original logarithm equation to make sense!

DPatrick
2015-10-07 20:08:17

So the only solution to our original equation is $x = \dfrac{1 + \sqrt5}{2}$, and thus there is $\boxed{1}$ solution.

So the only solution to our original equation is $x = \dfrac{1 + \sqrt5}{2}$, and thus there is $\boxed{1}$ solution.

DPatrick
2015-10-07 20:08:39

$e$ is a really cool number: it's arguably the most important number in mathematics. (I would vote for 0 myself.)

$e$ is a really cool number: it's arguably the most important number in mathematics. (I would vote for 0 myself.)

DPatrick
2015-10-07 20:08:51

One way to define $e$ is as the infinite sum of the reciprocals of all the factorials:

\[ e = \frac{1}{0!} + \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \cdots. \]

One way to define $e$ is as the infinite sum of the reciprocals of all the factorials:

\[ e = \frac{1}{0!} + \frac{1}{1!} + \frac{1}{2!} + \frac{1}{3!} + \cdots. \]

DPatrick
2015-10-07 20:09:06

Another way to define $e$ is to do the following thought experiment:

Another way to define $e$ is to do the following thought experiment:

DPatrick
2015-10-07 20:09:11

Imagine that the bank down the street has a savings account that pays $100\%$ annual interest! If I put $\$1$ in the bank at the start of the year, how much money will I have at the end of the year?

Imagine that the bank down the street has a savings account that pays $100\%$ annual interest! If I put $\$1$ in the bank at the start of the year, how much money will I have at the end of the year?

mathwhiz16
2015-10-07 20:09:27

$2.

$2.

zsp
2015-10-07 20:09:27

$2

$2

jxiao
2015-10-07 20:09:27

$2

$2

Destructio
2015-10-07 20:09:27

2

2

DPatrick
2015-10-07 20:09:34

True, if the interest is just compounded once -- that is, is just paid as a single lump sum at the end of the year -- then I'll receive $\$1$ in interest, and have a total of $\$2$ at the end of the year.

True, if the interest is just compounded once -- that is, is just paid as a single lump sum at the end of the year -- then I'll receive $\$1$ in interest, and have a total of $\$2$ at the end of the year.

DPatrick
2015-10-07 20:09:46

But what if the interest is compounded semi-annually: that is, once after 6 months and again at the end of the year?

But what if the interest is compounded semi-annually: that is, once after 6 months and again at the end of the year?

zsp
2015-10-07 20:10:03

$2.25

$2.25

mathwhiz16
2015-10-07 20:10:03

$2.25

$2.25

DPatrick
2015-10-07 20:10:09

Exactly. After 6 months, I'll get $50\%$ interest on my dollar, so I'll have $\$1.50$.

Exactly. After 6 months, I'll get $50\%$ interest on my dollar, so I'll have $\$1.50$.

DPatrick
2015-10-07 20:10:15

Then, at the end of the year, I'll get $50\%$ interest on my $\$1.50$. So that's another $\$0.75$ of interest, and I'll have $\$2.25$ at the end of the year.

Then, at the end of the year, I'll get $50\%$ interest on my $\$1.50$. So that's another $\$0.75$ of interest, and I'll have $\$2.25$ at the end of the year.

stronto
2015-10-07 20:10:32

It goes towards e ... doesn't it

It goes towards e ... doesn't it

quartzgirl
2015-10-07 20:10:32

this number zeroes in on e

this number zeroes in on e

DPatrick
2015-10-07 20:10:41

Indeed...let's see what happens if the interest is compounded monthly. Note that I get $\frac{100}{12}\%$ on my money at the end of each month. (I'm going to leave off the dollar signs, and round to 3 decimal places -- some of the totals might be off by 0.001 due to rounding.)

Indeed...let's see what happens if the interest is compounded monthly. Note that I get $\frac{100}{12}\%$ on my money at the end of each month. (I'm going to leave off the dollar signs, and round to 3 decimal places -- some of the totals might be off by 0.001 due to rounding.)

DPatrick
2015-10-07 20:10:46

\[\begin{array}{r|c|c}

\text{Month} & \text{Interest Earned} & \text{New Balance} \\ \hline

1 & 0.083 & 1.083 \\

2 & 0.090 & 1.174 \\

3 & 0.098 & 1.271 \\

4 & 0.106 & 1.377 \\

5 & 0.115 & 1.492 \\

6 & 0.124 & 1.616 \\

7 & 0.135 & 1.751 \\

8 & 0.146 & 1.897 \\

9 & 0.158 & 2.055 \\

10 & 0.171 & 2.226 \\

11 & 0.186 & 2.412 \\

12 & 0.201 & 2.613

\end{array}\]

\[\begin{array}{r|c|c}

\text{Month} & \text{Interest Earned} & \text{New Balance} \\ \hline

1 & 0.083 & 1.083 \\

2 & 0.090 & 1.174 \\

3 & 0.098 & 1.271 \\

4 & 0.106 & 1.377 \\

5 & 0.115 & 1.492 \\

6 & 0.124 & 1.616 \\

7 & 0.135 & 1.751 \\

8 & 0.146 & 1.897 \\

9 & 0.158 & 2.055 \\

10 & 0.171 & 2.226 \\

11 & 0.186 & 2.412 \\

12 & 0.201 & 2.613

\end{array}\]

DPatrick
2015-10-07 20:10:54

Now I've got a little over $\$2.61$ at the end of the year! The more often we compound, the better I make out.

Now I've got a little over $\$2.61$ at the end of the year! The more often we compound, the better I make out.

jxiao
2015-10-07 20:11:16

When time goes down infinitely small, you get $e.

When time goes down infinitely small, you get $e.

Harry0531
2015-10-07 20:11:16

The limit of this is $e$

The limit of this is $e$

EulerMacaroni
2015-10-07 20:11:16

approaches $e$

approaches $e$

DPatrick
2015-10-07 20:11:30

Right! Now imagine that my bank is compounding infinitely often! How much will I have at the end of the year?

Right! Now imagine that my bank is compounding infinitely often! How much will I have at the end of the year?

DPatrick
2015-10-07 20:11:33

I'll have exactly $\$e \approx \$2.71828$ in my account if the interest is compounded infinitely often.

I'll have exactly $\$e \approx \$2.71828$ in my account if the interest is compounded infinitely often.

DPatrick
2015-10-07 20:11:49

To say this in a formula: the formula for the amount of money that I'll have at the end of the year, if it is compounded $n$ times during the year, is

\[ \text{Amount} = \left(1 + \frac{1}{n}\right)^n. \]

To say this in a formula: the formula for the amount of money that I'll have at the end of the year, if it is compounded $n$ times during the year, is

\[ \text{Amount} = \left(1 + \frac{1}{n}\right)^n. \]

DPatrick
2015-10-07 20:11:59

For example, for the monthly compounding, the amount is

\[ \text{Amount} = \left(1 + \frac{1}{12}\right)^{12} = \left(\frac{13}{12}\right)^{12} \approx \frac{2.3298 \cdot 10^{13}}{8.9161 \cdot 10^{12}} \approx 2.613. \]

For example, for the monthly compounding, the amount is

\[ \text{Amount} = \left(1 + \frac{1}{12}\right)^{12} = \left(\frac{13}{12}\right)^{12} \approx \frac{2.3298 \cdot 10^{13}}{8.9161 \cdot 10^{12}} \approx 2.613. \]

DPatrick
2015-10-07 20:12:15

We get $e$ by letting $n$ get arbitrarily large. The specific mathematical term for this is a

\[ e = \lim_{n \rightarrow \infty} \left(1 + \frac{1}{n}\right)^n. \]

We get $e$ by letting $n$ get arbitrarily large. The specific mathematical term for this is a

**limit**:\[ e = \lim_{n \rightarrow \infty} \left(1 + \frac{1}{n}\right)^n. \]

Harry0531
2015-10-07 20:12:43

Actually, we define $e$ to be the number where $e^x$'s derivitive is itself

Actually, we define $e$ to be the number where $e^x$'s derivitive is itself

DPatrick
2015-10-07 20:12:48

Right. Two other amazing facts about $e$ before we go on.

Right. Two other amazing facts about $e$ before we go on.

DPatrick
2015-10-07 20:12:51

First, if you know a little calculus, you'll undoubtedly know the significance of $e$: the functions $f(x) = ce^x$ (for some constant $c$) are the only real-valued functions that equal their own derivative. That is, the exponential function with base $e$, up to a constant multiple, is the only solution to the differential equation $y' = y$.

First, if you know a little calculus, you'll undoubtedly know the significance of $e$: the functions $f(x) = ce^x$ (for some constant $c$) are the only real-valued functions that equal their own derivative. That is, the exponential function with base $e$, up to a constant multiple, is the only solution to the differential equation $y' = y$.

DPatrick
2015-10-07 20:13:01

(If you don't know what this means, don't worry -- you'll learn it in calculus class!)

(If you don't know what this means, don't worry -- you'll learn it in calculus class!)

atmchallenge
2015-10-07 20:13:09

$e^{\pi i}+1=0$

$e^{\pi i}+1=0$

DPatrick
2015-10-07 20:13:20

Second, Euler (there he is again!) discovered the fundamental relationship between $e$, complex number, and trigonometric functions, in what is today called

\[ e^{i\theta} = \cos \theta + i \sin \theta.\]

Second, Euler (there he is again!) discovered the fundamental relationship between $e$, complex number, and trigonometric functions, in what is today called

**Euler's Formula**:\[ e^{i\theta} = \cos \theta + i \sin \theta.\]

DPatrick
2015-10-07 20:13:34

If we set $\theta$ equal to $\pi$, we get what was voted the most beautiful formula in mathematics:

\[ e^{i \pi} + 1 = 0. \]

This formula combines the five most important numbers in mathematics: 0, 1, $i$, $\pi$, and $e$.

If we set $\theta$ equal to $\pi$, we get what was voted the most beautiful formula in mathematics:

\[ e^{i \pi} + 1 = 0. \]

This formula combines the five most important numbers in mathematics: 0, 1, $i$, $\pi$, and $e$.

DPatrick
2015-10-07 20:14:01

There are a gazillion more facts about $e$ I could talk about, but let's move onwards!

There are a gazillion more facts about $e$ I could talk about, but let's move onwards!

DPatrick
2015-10-07 20:14:10

8. A prism has an equilateral triangle as its base and top and three rectangles for its lateral sides. A sphere of radius 1 fits inside the prism, touching all five of its faces. What is the volume of the prism?

8. A prism has an equilateral triangle as its base and top and three rectangles for its lateral sides. A sphere of radius 1 fits inside the prism, touching all five of its faces. What is the volume of the prism?

Wave-Particle
2015-10-07 20:14:32

diagram

diagram

Pimaster314
2015-10-07 20:14:32

picture

picture

DPatrick
2015-10-07 20:14:33

Here's a crude picture of our prism (without the sphere drawn):

Here's a crude picture of our prism (without the sphere drawn):

DPatrick
2015-10-07 20:14:36

DPatrick
2015-10-07 20:14:44

What's the formula for the volume of a prism?

What's the formula for the volume of a prism?

Liopleurodon
2015-10-07 20:15:07

BH

BH

trumpeter
2015-10-07 20:15:07

$Bh$

$Bh$

matl26
2015-10-07 20:15:07

bh

bh

atmchallenge
2015-10-07 20:15:07

$V=bh$

$V=bh$

runikmehrotra
2015-10-07 20:15:07

Base time height

Base time height

mihirb
2015-10-07 20:15:07

Area of the base times the height

Area of the base times the height

bguo
2015-10-07 20:15:07

base area*height

base area*height

szhang7853
2015-10-07 20:15:07

base times height

base times height

aYummyNoodle
2015-10-07 20:15:07

base times height

base times height

nosaj
2015-10-07 20:15:07

$Bh$

$Bh$

eveningstarandlion
2015-10-07 20:15:07

triangle area * height

triangle area * height

DPatrick
2015-10-07 20:15:13

Yep, it's just the area of the base times the height.

Yep, it's just the area of the base times the height.

DPatrick
2015-10-07 20:15:19

Do we know either of these right away?

Do we know either of these right away?

nosaj
2015-10-07 20:15:41

Note that the height of the prism is twice the radius of the sphere.

Note that the height of the prism is twice the radius of the sphere.

ninjataco
2015-10-07 20:15:41

height = 2

height = 2

goodbear
2015-10-07 20:15:41

h=2

h=2

mathman2048
2015-10-07 20:15:41

the height is 2

the height is 2

azmath333
2015-10-07 20:15:41

$h=2$

$h=2$

int_user
2015-10-07 20:15:41

height is 2

height is 2

rt03
2015-10-07 20:15:41

h=2r=2 because the sphere fits inside the prism

h=2r=2 because the sphere fits inside the prism

ohmcfifth
2015-10-07 20:15:41

Height = 2

Height = 2

DPatrick
2015-10-07 20:15:46

Right. The sphere fits snugly inside the prism and touches all 5 faces.

Right. The sphere fits snugly inside the prism and touches all 5 faces.

DPatrick
2015-10-07 20:15:50

So in particular, the diameter of the sphere is the height of the prism.

So in particular, the diameter of the sphere is the height of the prism.

DPatrick
2015-10-07 20:16:01

But we're given that the radius is 1, so the diameter is 2, and thus the height is 2.

But we're given that the radius is 1, so the diameter is 2, and thus the height is 2.

DPatrick
2015-10-07 20:16:07

So if we can find the area of the equilateral triangle base, then we'll be done.

So if we can find the area of the equilateral triangle base, then we'll be done.

trumpeter
2015-10-07 20:16:27

take a cross section

take a cross section

mihirb
2015-10-07 20:16:27

We should make a cross section

We should make a cross section

mihirb
2015-10-07 20:16:42

the radius of the sphere is the inradius of the triangle

the radius of the sphere is the inradius of the triangle

nosaj
2015-10-07 20:16:42

Take a cross section parallel to the base and passing through the center of the sphere.

Take a cross section parallel to the base and passing through the center of the sphere.

atmchallenge
2015-10-07 20:16:42

The great circle of the sphere is the equilateral triangle's incircle.

The great circle of the sphere is the equilateral triangle's incircle.

DPatrick
2015-10-07 20:16:51

Right. The sphere is also tangent to the three sides. So if we cut a cross-section through the middle of the prism, we'll get a circle of radius 1 that's tangent to all three sides of an equilateral triangle:

Right. The sphere is also tangent to the three sides. So if we cut a cross-section through the middle of the prism, we'll get a circle of radius 1 that's tangent to all three sides of an equilateral triangle:

DPatrick
2015-10-07 20:16:54

DPatrick
2015-10-07 20:17:13

How can we compute the area of this triangle, given that we know the radius of the circle is 1?

How can we compute the area of this triangle, given that we know the radius of the circle is 1?

Pimaster314
2015-10-07 20:17:37

inradius

inradius

letsgomath
2015-10-07 20:17:37

inradius formula

inradius formula

lkarhat
2015-10-07 20:17:37

inradius formula

inradius formula

bguo
2015-10-07 20:17:37

incircle formula

incircle formula

DPatrick
2015-10-07 20:17:47

There's a formula for this, but I like to keep it simple.

There's a formula for this, but I like to keep it simple.

mathman2048
2015-10-07 20:17:50

make a 30 60 90 with the radius and the left side

make a 30 60 90 with the radius and the left side

DPatrick
2015-10-07 20:18:02

Yes, I'd just draw in a 30-60-90 triangle:

Yes, I'd just draw in a 30-60-90 triangle:

DPatrick
2015-10-07 20:18:05

DPatrick
2015-10-07 20:18:14

What's the area of the blue triangle?

What's the area of the blue triangle?

jxiao
2015-10-07 20:18:37

sqrt(3)/2

sqrt(3)/2

eveningstarandlion
2015-10-07 20:18:37

sqrt3/2

sqrt3/2

stronto
2015-10-07 20:18:37

sqrt3 / 2

sqrt3 / 2

ImpossibleCube
2015-10-07 20:18:37

$\sqrt{3}/2$

$\sqrt{3}/2$

zsp
2015-10-07 20:18:37

$\frac{\sqrt{3}}{2}$

$\frac{\sqrt{3}}{2}$

Alpha-aops
2015-10-07 20:18:37

sqrt3/2

sqrt3/2

mathwhiz16
2015-10-07 20:18:37

$\sqrt{3}/2$

$\sqrt{3}/2$

DPatrick
2015-10-07 20:18:45

It's a right triangle with legs of length $1$ and $\sqrt3$, so its area is $\frac{\sqrt3}{2}$.

It's a right triangle with legs of length $1$ and $\sqrt3$, so its area is $\frac{\sqrt3}{2}$.

szhang7853
2015-10-07 20:18:52

1/6 of the area of the triangle

1/6 of the area of the triangle

sturdyoak2012
2015-10-07 20:18:55

multiply by 6

multiply by 6

DPatrick
2015-10-07 20:19:03

And yes, the entire triangle is just six of these smaller blue triangles:

And yes, the entire triangle is just six of these smaller blue triangles:

DPatrick
2015-10-07 20:19:07

Alpha-aops
2015-10-07 20:19:20

3sqrt3

3sqrt3

int_user
2015-10-07 20:19:20

so 3sqrt3

so 3sqrt3

mathwhiz16
2015-10-07 20:19:20

so $3sqrt{3}$

so $3sqrt{3}$

eveningstarandlion
2015-10-07 20:19:28

the triangle is $3\sqrt{3}$

the triangle is $3\sqrt{3}$

rt03
2015-10-07 20:19:28

Area of base is $(6)(\frac{\sqrt{3}}{2}=3\sqrt{3}$.

Area of base is $(6)(\frac{\sqrt{3}}{2}=3\sqrt{3}$.

DPatrick
2015-10-07 20:19:29

So the triangle has area $6 \cdot \frac{\sqrt3}{2} = 3\sqrt3$.

So the triangle has area $6 \cdot \frac{\sqrt3}{2} = 3\sqrt3$.

runikmehrotra
2015-10-07 20:19:48

so the volume is 3sqrt(3) *2

so the volume is 3sqrt(3) *2

ninjataco
2015-10-07 20:19:48

so the volume is $6\sqrt{3}$

so the volume is $6\sqrt{3}$

runikmehrotra
2015-10-07 20:19:48

or 6sqrt(3)

or 6sqrt(3)

DPatrick
2015-10-07 20:19:54

And to finish up, the base area is $3\sqrt3$ and the height is 2, so the volume is their product, $\boxed{6\sqrt3}$.

And to finish up, the base area is $3\sqrt3$ and the height is 2, so the volume is their product, $\boxed{6\sqrt3}$.

DPatrick
2015-10-07 20:20:14

Nearing the end of the contest -- on to #9:

Nearing the end of the contest -- on to #9:

DPatrick
2015-10-07 20:20:18

9. Bob and Jane have three children. Given that one child is their daughter Mary, what is the probability that Bob and Jane have at least two daughters?

9. Bob and Jane have three children. Given that one child is their daughter Mary, what is the probability that Bob and Jane have at least two daughters?

trumpeter
2015-10-07 20:20:36

were there two possible answers to this, depending on interpretation?

were there two possible answers to this, depending on interpretation?

DPatrick
2015-10-07 20:20:43

Indeed...

Indeed...

DPatrick
2015-10-07 20:21:08

The

The

*Who Wants to Be a Mathematician*staff have decided that, due to the ambiguity of the problem, they are accepting two different answers.
DPatrick
2015-10-07 20:21:33

One interpretation of this problem is as an example of

One interpretation of this problem is as an example of

**conditional probability**.
DPatrick
2015-10-07 20:21:51

That is, we're asked to find the probability that they have at least 2 daughters, given that they have at least 1 daughter.

That is, we're asked to find the probability that they have at least 2 daughters, given that they have at least 1 daughter.

mathwhiz16
2015-10-07 20:22:11

Count the number of possible cases and the number of cases with two daughters.

Count the number of possible cases and the number of cases with two daughters.

DPatrick
2015-10-07 20:22:31

Exactly. We can list all the possibilities for 3 children in a chart:

Exactly. We can list all the possibilities for 3 children in a chart:

DPatrick
2015-10-07 20:22:34

\[\begin{array}{c|c|c}

\text{Oldest} & \text{Middle} & \text{Youngest} \\ \hline

B&B&B \\

B&B&G \\

B&G&B \\

G&B&B \\

B&G&G \\

G&B&G \\

G&G&B \\

G&G&G

\end{array}\]

\[\begin{array}{c|c|c}

\text{Oldest} & \text{Middle} & \text{Youngest} \\ \hline

B&B&B \\

B&B&G \\

B&G&B \\

G&B&B \\

B&G&G \\

G&B&G \\

G&G&B \\

G&G&G

\end{array}\]

DPatrick
2015-10-07 20:22:58

But because we are told that the couple has a daughter, we can eliminate the top row, leaving 7 equally-likely possibilities:

But because we are told that the couple has a daughter, we can eliminate the top row, leaving 7 equally-likely possibilities:

DPatrick
2015-10-07 20:23:06

\[\begin{array}{c|c|c}

\text{Oldest} & \text{Middle} & \text{Youngest} \\ \hline

B&B&G \\

B&G&B \\

G&B&B \\

B&G&G \\

G&B&G \\

G&G&B \\

G&G&G

\end{array}\]

\[\begin{array}{c|c|c}

\text{Oldest} & \text{Middle} & \text{Youngest} \\ \hline

B&B&G \\

B&G&B \\

G&B&B \\

B&G&G \\

G&B&G \\

G&G&B \\

G&G&G

\end{array}\]

mathwhiz16
2015-10-07 20:23:21

7 cases, 4 with two daughters.

7 cases, 4 with two daughters.

DPatrick
2015-10-07 20:23:31

Right. The bottom four rows are the possibilities with at least two girls, so the probability of that occurring is $\boxed{\frac47}$.

Right. The bottom four rows are the possibilities with at least two girls, so the probability of that occurring is $\boxed{\frac47}$.

DPatrick
2015-10-07 20:23:43

However...

However...

DPatrick
2015-10-07 20:23:48

The fact of the matter is, English is a lot less precise than math. And as such, problems like this are open to different interpretations depending on how you interpret the English.

The fact of the matter is, English is a lot less precise than math. And as such, problems like this are open to different interpretations depending on how you interpret the English.

DPatrick
2015-10-07 20:23:56

A reasonable alternative interpretation of the problem would be:

A reasonable alternative interpretation of the problem would be:

DPatrick
2015-10-07 20:24:01

9. Bob and Jane have three children. Given that one child is their daughter Mary, what is the probability that at least one of their other children is a girl?

9. Bob and Jane have three children. Given that one child is their daughter Mary, what is the probability that at least one of their other children is a girl?

DPatrick
2015-10-07 20:24:25

Which basically reads the same way in English, but now it's not a conditional probability problem at all: it's just a "usual" probability problem with no condition.

Which basically reads the same way in English, but now it's not a conditional probability problem at all: it's just a "usual" probability problem with no condition.

goodbear
2015-10-07 20:24:36

3/4

3/4

Liopleurodon
2015-10-07 20:24:36

Ohhhhhh! so it would be 3/4

Ohhhhhh! so it would be 3/4

Knin2820
2015-10-07 20:24:36

There's a $\frac{1}{2}$ chance of having a boy. So there's a $\frac{1}{4}$ chance of having 2 boys, and no girls. Therefore, there is a $1-\frac{1}{4}=\frac{3}{4}$ chance of having at least one more girl.

There's a $\frac{1}{2}$ chance of having a boy. So there's a $\frac{1}{4}$ chance of having 2 boys, and no girls. Therefore, there is a $1-\frac{1}{4}=\frac{3}{4}$ chance of having at least one more girl.

DPatrick
2015-10-07 20:24:56

Right, that's one solution: they can only not have a girl (among the two other children) if the two other kids are both boys. This occurs with probability $\left(\frac12\right)^2 = \frac14$. Therefore, the probability that, among two children, at least one is a girl, is $1 - \frac14 = \boxed{\frac34}$.

Right, that's one solution: they can only not have a girl (among the two other children) if the two other kids are both boys. This occurs with probability $\left(\frac12\right)^2 = \frac14$. Therefore, the probability that, among two children, at least one is a girl, is $1 - \frac14 = \boxed{\frac34}$.

DPatrick
2015-10-07 20:25:11

Or, we could make a chart for the two other children:

\[\begin{array}{c|c}

\text{Older} & \text{Younger} \\ \hline

B & B \\

B & G \\

G & B \\

G & G

\end{array}\]

Or, we could make a chart for the two other children:

\[\begin{array}{c|c}

\text{Older} & \text{Younger} \\ \hline

B & B \\

B & G \\

G & B \\

G & G

\end{array}\]

DPatrick
2015-10-07 20:25:16

There are 4 equally-likely rows, and 3 of these rows have at least one girl. Therefore, the probability of at least one girl is $\boxed{\frac34}$.

There are 4 equally-likely rows, and 3 of these rows have at least one girl. Therefore, the probability of at least one girl is $\boxed{\frac34}$.

EulerMacaroni
2015-10-07 20:25:26

this problem is a more general version of a well-known paradox

this problem is a more general version of a well-known paradox

DPatrick
2015-10-07 20:25:36

Right you are. There's a lengthy discussion of this sort of problem on Wikipedia at

http://en.wikipedia.org/wiki/Boy_or_Girl_paradox

Right you are. There's a lengthy discussion of this sort of problem on Wikipedia at

http://en.wikipedia.org/wiki/Boy_or_Girl_paradox

DPatrick
2015-10-07 20:25:46

It's an unfortunate fact of life, and of the imprecision of the English language, that problems like this are usually ambiguous and open to multiple interpretations.

It's an unfortunate fact of life, and of the imprecision of the English language, that problems like this are usually ambiguous and open to multiple interpretations.

DPatrick
2015-10-07 20:25:53

Though I wouldn't call it a "paradox" -- rather, it's a consequence of the imprecision of the English language.

Though I wouldn't call it a "paradox" -- rather, it's a consequence of the imprecision of the English language.

TheLuckyAngelo
2015-10-07 20:26:00

The Monty Hall Problem!!!!

The Monty Hall Problem!!!!

DPatrick
2015-10-07 20:26:04

This is also somewhat related to the famous "Monty Hall" problem. If you haven't heard of that before, I highly recommend looking it up.

This is also somewhat related to the famous "Monty Hall" problem. If you haven't heard of that before, I highly recommend looking it up.

DPatrick
2015-10-07 20:26:14

And as I said at the beginning of this problem, the

And as I said at the beginning of this problem, the

*Who Wants to Be a Mathematician*staff have decided that, due to the ambiguity of the problem, they are accepting both $\frac47$ and $\frac34$ as correct answers.
DPatrick
2015-10-07 20:26:54

And on to the finish at #10, which was actually a bit easier (in my opinion) than some of the earlier problem:

And on to the finish at #10, which was actually a bit easier (in my opinion) than some of the earlier problem:

DPatrick
2015-10-07 20:26:58

10. What integer is closest to the square root of the product of the largest two-digit prime number and the smallest three-digit prime number?

10. What integer is closest to the square root of the product of the largest two-digit prime number and the smallest three-digit prime number?

szhang7853
2015-10-07 20:27:46

the largest 2 digit prime is 97 and the smallest 3 digit prime is 101

the largest 2 digit prime is 97 and the smallest 3 digit prime is 101

mathman2048
2015-10-07 20:27:46

97 x 101 square rooted

97 x 101 square rooted

runikmehrotra
2015-10-07 20:27:46

isnt it just 97 times 101

isnt it just 97 times 101

DeathLlama9
2015-10-07 20:27:46

$97 \times 101$

$97 \times 101$

phi_ftw1618
2015-10-07 20:27:46

So closest to $\sqrt{97\cdot101}$?

So closest to $\sqrt{97\cdot101}$?

bearytasty
2015-10-07 20:27:46

sqrt(97*101)

sqrt(97*101)

Liopleurodon
2015-10-07 20:27:46

97*101

97*101

Harry0531
2015-10-07 20:27:46

$97\times 101$

$97\times 101$

bearytasty
2015-10-07 20:27:46

97 and 101 are the two numbers

97 and 101 are the two numbers

ninjataco
2015-10-07 20:27:46

97 and 101

97 and 101

DPatrick
2015-10-07 20:28:04

Right. The largest two-digit prime is 97, and the smallest three-digit prime is 101.

Right. The largest two-digit prime is 97, and the smallest three-digit prime is 101.

DPatrick
2015-10-07 20:28:08

So we're looking for the integer that's closest to $\sqrt{97 \cdot 101}$.

So we're looking for the integer that's closest to $\sqrt{97 \cdot 101}$.

Alpha-aops
2015-10-07 20:28:42

99

99

djduan
2015-10-07 20:28:42

99

99

EulerMacaroni
2015-10-07 20:28:42

so like 99

so like 99

SilverPersian1
2015-10-07 20:28:42

99

99

rt03
2015-10-07 20:28:42

Start around 99 and estimate numbers that are $\sqrt{97 \times 101}$.

Start around 99 and estimate numbers that are $\sqrt{97 \times 101}$.

WL0410
2015-10-07 20:28:49

$\sqrt{(99+2)(99-2)}=\sqrt{99^2-4}$

$\sqrt{(99+2)(99-2)}=\sqrt{99^2-4}$

trumpeter
2015-10-07 20:28:49

$97\cdot101=99^2-4$

$97\cdot101=99^2-4$

DPatrick
2015-10-07 20:28:54

It's very tempting to say 99.

It's very tempting to say 99.

DPatrick
2015-10-07 20:28:59

Note that if $x = 99$, then our quantity is $\sqrt{(x-2)(x+2)}$, which simplifies to $\sqrt{x^2-4}$.

Note that if $x = 99$, then our quantity is $\sqrt{(x-2)(x+2)}$, which simplifies to $\sqrt{x^2-4}$.

DPatrick
2015-10-07 20:29:09

It seems very likely that if $x$ is a large enough integer, then $\sqrt{x^2 - 4}$ is closest to the integer $\sqrt{x^2} = x$. Is 99 large enough for this to happen?

It seems very likely that if $x$ is a large enough integer, then $\sqrt{x^2 - 4}$ is closest to the integer $\sqrt{x^2} = x$. Is 99 large enough for this to happen?

zsp
2015-10-07 20:29:29

Yes

Yes

letsgomath
2015-10-07 20:29:29

yes

yes

dtxiong
2015-10-07 20:29:29

yes

yes

eveningstarandlion
2015-10-07 20:29:29

Yes

Yes

Harry0531
2015-10-07 20:29:29

Itwould seem so...

Itwould seem so...

DPatrick
2015-10-07 20:29:33

Absolutely!

Absolutely!

DPatrick
2015-10-07 20:29:38

We just need $x^2-4$ to be between $\left(x-\frac12\right)^2$ and $x^2$.

We just need $x^2-4$ to be between $\left(x-\frac12\right)^2$ and $x^2$.

DPatrick
2015-10-07 20:29:46

But $\left(x-\frac12\right)^2 = x^2 - x + \frac14$.

But $\left(x-\frac12\right)^2 = x^2 - x + \frac14$.

DPatrick
2015-10-07 20:29:56

So as long as $x^2 - x + \frac14 < x^2 - 4$, then $\sqrt{x^2-4}$ will round to $x$.

So as long as $x^2 - x + \frac14 < x^2 - 4$, then $\sqrt{x^2-4}$ will round to $x$.

DPatrick
2015-10-07 20:30:16

Thus all we need is $-x < -\frac{17}{4}$, or $x > \frac{17}{4}$. That's clearly the case if $x=99$, so we're done.

Thus all we need is $-x < -\frac{17}{4}$, or $x > \frac{17}{4}$. That's clearly the case if $x=99$, so we're done.

DeathLlama9
2015-10-07 20:30:22

So $x > \frac{15}{4}$, and since $99$ is a lot bigger than $3.75$ it works

So $x > \frac{15}{4}$, and since $99$ is a lot bigger than $3.75$ it works

DPatrick
2015-10-07 20:30:28

Right. The answer is $\boxed{99}$.

Right. The answer is $\boxed{99}$.

DPatrick
2015-10-07 20:30:36

That's it for Round 1!

That's it for Round 1!

DPatrick
2015-10-07 20:30:45

If you officially participated in the contest, and you got at least 8 of these 10 problems correct, then you'll move on to Round 2 later this month. Official notification of this should go out to your teacher later this week.

If you officially participated in the contest, and you got at least 8 of these 10 problems correct, then you'll move on to Round 2 later this month. Official notification of this should go out to your teacher later this week.

DPatrick
2015-10-07 20:30:55

We will have a Round 2 Math Jam on Tuesday, November 3 at 7:30 PM Eastern / 4:30 PM Pacific to discuss the Round 2 problems and solutions.

We will have a Round 2 Math Jam on Tuesday, November 3 at 7:30 PM Eastern / 4:30 PM Pacific to discuss the Round 2 problems and solutions.

DPatrick
2015-10-07 20:31:09

They'll be, on average, a bit harder.

They'll be, on average, a bit harder.

DPatrick
2015-10-07 20:31:20

Then, the top scorer on Round 2 in each of 9 regions, plus the top scorer in the Seattle area, will advance to the National semifinals, to be held live at the 2016 Joint Mathematics Meetings in Seattle in January. Perhaps I'll see you there!

Then, the top scorer on Round 2 in each of 9 regions, plus the top scorer in the Seattle area, will advance to the National semifinals, to be held live at the 2016 Joint Mathematics Meetings in Seattle in January. Perhaps I'll see you there!

szhang7853
2015-10-07 20:31:38

what are the regions?

what are the regions?

Liopleurodon
2015-10-07 20:31:38

why is the Seattle region special?

why is the Seattle region special?

DPatrick
2015-10-07 20:31:49

There's a map of the regions on the website (which is coming in a moment).

There's a map of the regions on the website (which is coming in a moment).

DPatrick
2015-10-07 20:31:56

Seattle is special because that's where the live finals will be.

Seattle is special because that's where the live finals will be.

DPatrick
2015-10-07 20:32:01

Visit http://www.ams.org/programs/students/wwtbam/wwtbam to learn more about the game and to visit the archive of past years' contests.

Visit http://www.ams.org/programs/students/wwtbam/wwtbam to learn more about the game and to visit the archive of past years' contests.

DPatrick
2015-10-07 20:32:19

There are also going to be regional versions of the game this spring in Rhode Island, Connecticut, and Washington, DC. Local teachers will be contacted about these regional games. Check out the above website for more info.

There are also going to be regional versions of the game this spring in Rhode Island, Connecticut, and Washington, DC. Local teachers will be contacted about these regional games. Check out the above website for more info.

mikebreen
2015-10-07 20:32:23

If you didn't participate in this year's test, we invite all of your teachers to email us at paoffice@ams.org and we'll put you on next year's mailing list.

If you didn't participate in this year's test, we invite all of your teachers to email us at paoffice@ams.org and we'll put you on next year's mailing list.

Snowie
2015-10-07 20:32:36

Will there be a transcript of this math jam?

Will there be a transcript of this math jam?

DPatrick
2015-10-07 20:32:45

Yes, the transcript will be available later this evening on the AoPS website.

Yes, the transcript will be available later this evening on the AoPS website.

DPatrick
2015-10-07 20:33:04

That's it for tonight's Math Jam. Thanks for coming!

That's it for tonight's Math Jam. Thanks for coming!

jxiao
2015-10-07 20:35:17

If the top scorer ties, what is the tiebreaker?

If the top scorer ties, what is the tiebreaker?

DPatrick
2015-10-07 20:35:41

Give me just a second -- this info is on the AMS website and I'll cut-and-paste it over here.

Give me just a second -- this info is on the AMS website and I'll cut-and-paste it over here.

DPatrick
2015-10-07 20:36:02

Tie-breakers. Because there are so few semifinal slots available for the national game and so many good students across the country, we often have many ties for first in a region, even many with perfect scores. In addition to the scores, further considerations for contest selection are gender (an effort is made to include an equal number of males and females); grade (a high school senior receives preference over younger students who will have another chance before they graduate); the correct answer on a particular qualifying test question (varies from year to year, usually question 10); and the free-form answers to the non-math questions.

- See more at: http://www.ams.org/programs/students/wwtbam/qualify#ties

Tie-breakers. Because there are so few semifinal slots available for the national game and so many good students across the country, we often have many ties for first in a region, even many with perfect scores. In addition to the scores, further considerations for contest selection are gender (an effort is made to include an equal number of males and females); grade (a high school senior receives preference over younger students who will have another chance before they graduate); the correct answer on a particular qualifying test question (varies from year to year, usually question 10); and the free-form answers to the non-math questions.

- See more at: http://www.ams.org/programs/students/wwtbam/qualify#ties

DPatrick
2015-10-07 20:36:33

And here's the regional map:

And here's the regional map:

DPatrick
2015-10-07 20:36:36

Rubaiya
2015-10-07 20:36:48

How do you register? Is it done by teams or individuals?

How do you register? Is it done by teams or individuals?

DPatrick
2015-10-07 20:37:06

It's through your school. Your teacher should email paoffice@ams.org for information.

It's through your school. Your teacher should email paoffice@ams.org for information.

1-1 is 3
2015-10-07 20:37:48

Where is a good place to learn all the history stuff for the contest?

Where is a good place to learn all the history stuff for the contest?

DPatrick
2015-10-07 20:38:01

There are a lot of websites devoted to mathematical history.

There are a lot of websites devoted to mathematical history.

DPatrick
2015-10-07 20:38:28

My favorite is the one I mentioned earlier: the MacTutor History of Mathematics archive at the University of St. Andrews in Scotland. The main page is http://www-groups.dcs.st-and.ac.uk/~history.

My favorite is the one I mentioned earlier: the MacTutor History of Mathematics archive at the University of St. Andrews in Scotland. The main page is http://www-groups.dcs.st-and.ac.uk/~history.

guluguluga
2015-10-07 20:38:33

How many history questions are there, usually?

How many history questions are there, usually?

DPatrick
2015-10-07 20:38:38

Just 1 out of the 10.

Just 1 out of the 10.

nosaj
2015-10-07 20:38:45

One student from each region gets to go to semifinals in seattle?

One student from each region gets to go to semifinals in seattle?

DPatrick
2015-10-07 20:38:52

That's right. Plus one student from the Seattle area.

That's right. Plus one student from the Seattle area.

guluguluga
2015-10-07 20:39:40

Does our teacher have to administer the test to us in a specific date?

Does our teacher have to administer the test to us in a specific date?

DPatrick
2015-10-07 20:40:06

There is a 2-week (I think) window, which for Round 1 was the last two weeks.

There is a 2-week (I think) window, which for Round 1 was the last two weeks.

DPatrick
2015-10-07 20:40:16

Round 2 is 10/17-11/2.

Round 2 is 10/17-11/2.

mikebreen
2015-10-07 20:40:27

Yes, any time in the two-week window.

Yes, any time in the two-week window.

Rubaiya
2015-10-07 20:40:48

Are there options for homeschooled students?

Are there options for homeschooled students?

DPatrick
2015-10-07 20:40:57

I'll let Mike answer this because I'm not sure.

I'll let Mike answer this because I'm not sure.

mikebreen
2015-10-07 20:41:26

Homeschooled students are eligible. We only require that the test is administered by whomever does the teaching at home.

Homeschooled students are eligible. We only require that the test is administered by whomever does the teaching at home.

kungfugirl
2015-10-07 20:41:34

Does the admission process require anything fancy?

Does the admission process require anything fancy?

DPatrick
2015-10-07 20:41:52

Nope -- just write to the email address that we provided a few minutes ago.

Nope -- just write to the email address that we provided a few minutes ago.

mikebreen
2015-10-07 20:42:14

No registration fee either.

No registration fee either.

Ciscool
2015-10-07 20:42:52

How do you check score?

How do you check score?

DPatrick
2015-10-07 20:43:03

I'll defer to Mike on the scoring -- he's the one in charge of it.

I'll defer to Mike on the scoring -- he's the one in charge of it.

mikebreen
2015-10-07 20:43:34

Teachers can see their students' scores online, via Maple TA.

Teachers can see their students' scores online, via Maple TA.

DPatrick
2015-10-07 20:44:34

OK, I think we're going to close down for the evening, which means we'll be closing the classroom very shortly. Thanks again for coming -- I hope you enjoyed it!

OK, I think we're going to close down for the evening, which means we'll be closing the classroom very shortly. Thanks again for coming -- I hope you enjoyed it!

mikebreen
2015-10-07 20:45:23

Loved it! Thanks.

Loved it! Thanks.

TPiR
2015-10-07 20:45:53

Terrific. Thanks.

Terrific. Thanks.