## Who Wants to Be a Mathematician, Qualifying Round

Go back to the Math Jam ArchiveAoPS instructor David Patrick will discuss the problems on Round 1 of the 2017-2018 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 San Diego in January 2018.

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#### Facilitator: Dave Patrick

DPatrick
2017-09-27 19:30:37

**Welcome to the 2017-18***Who Wants to Be a Mathematician*Round 1 Math Jam!
DPatrick
2017-09-27 19:30:48

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 13 years, and I've written or co-written a few of our textbooks.

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 13 years, and I've written or co-written a few of our textbooks.

DPatrick
2017-09-27 19:31:01

In keeping with tonight's theme, I also once was a contestant on ABC's

In keeping with tonight's theme, I also once was a contestant on ABC's

*Who Wants to Be a Millionaire*back before I started working at AoPS, way back when Regis Philbin was still the host. Here's a picture (I'm on the left, Regis is on the right):
DPatrick
2017-09-27 19:31:07

DPatrick
2017-09-27 19:31:24

*Photo Credit: Maria Melin, copyright 1999 ABC Television.*
DPatrick
2017-09-27 19:31:40

No, I didn't win the million bucks, but I did win enough to buy a new car.

No, I didn't win the million bucks, but I did win enough to buy a new car.

DPatrick
2017-09-27 19:31:46

Before we get started I would like to take a moment to explain our virtual classroom procedures to those who have not previously participated in a Math Jam or one of our online classes.

Before we get started I would like to take a moment to explain our virtual classroom procedures to those who have not previously participated in a Math Jam or one of our online classes.

DPatrick
2017-09-27 19:32:01

The classroom is

The classroom is

**moderated**, meaning that participants 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
2017-09-27 19:32:12

This helps keep the class organized and on track. This also means that only

This helps keep the class organized and on track. This also means that only

**well-written**comments will be dropped into the classroom, so please take time writing responses that are complete and easy to read.
DPatrick
2017-09-27 19:32:43

There are a lot of students here! As such, only a fraction of the 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! As such, only a fraction of the 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
2017-09-27 19:33:00

Also, we won't be going through 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 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
2017-09-27 19:33:17

We have an assistant here to help out tonight: Karthik Karnik (

We have an assistant here to help out tonight: Karthik Karnik (

**Pythagor25**). Karthik is a sophomore at Caltech who enjoys solving puzzles, playing chess, programming UAVs, and robotics. He is majoring in math and computer science.
DPatrick
2017-09-27 19:33:24

He can try to help you if you have a question or are having some other difficulty. He may open a private window with you to chat if needed.

He can try to help you if you have a question or are having some other difficulty. He may open a private window with you to chat if needed.

DPatrick
2017-09-27 19:33:52

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

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

**mikebreen**) and Bill Butterworth (**TPiR**).
mikebreen
2017-09-27 19:34:03

Hello, everyone. Thanks for joining in.

Hello, everyone. Thanks for joining in.

DPatrick
2017-09-27 19:34:06

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.
TPiR
2017-09-27 19:34:16

Hi everyone!

Hi everyone!

DPatrick
2017-09-27 19:34:20

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
2017-09-27 19:35:05

Thanks very much to Mike and Bill for joining us tonight. Mike will also be around to answer questions (as will I) after the Math Jam has concluded. As you can see, we have a lot of game show background here tonight!

Thanks very much to Mike and Bill for joining us tonight. Mike will also be around to answer questions (as will I) after the Math Jam has concluded. As you can see, we have a lot of game show background here tonight!

DPatrick
2017-09-27 19:35:30

*Who Wants to Be a Mathematician*is a math contest for high school students 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.
Zachdog1
2017-09-27 19:35:46

What is the age requirement?

What is the age requirement?

DPatrick
2017-09-27 19:36:06

There isn't one. Even though the contest is at the high school level, anyone high school or below can participate.

There isn't one. Even though the contest is at the high school level, anyone high school or below can participate.

DPatrick
2017-09-27 19:36:26

Tonight we'll be talking about Round 1 of the national contest, which just concluded. It consisted of 10 questions, with a 15-minute time limit. So the problems are quick: you have an average of 90 seconds per question. (But as we'll see as we work through the problems, some of them shouldn't take you nearly that long.) No books, notes, calculators, or internet were permitted during the test.

Tonight we'll be talking about Round 1 of the national contest, which just concluded. It consisted of 10 questions, with a 15-minute time limit. So the problems are quick: you have an average of 90 seconds per question. (But as we'll see as we work through the problems, some of them shouldn't take you nearly that long.) No books, notes, calculators, or internet were permitted during the test.

yeskay
2017-09-27 19:36:49

You can have scratch paper right?

You can have scratch paper right?

DPatrick
2017-09-27 19:36:51

Yes.

Yes.

GeronimoStilton
2017-09-27 19:36:55

I took it online though.

I took it online though.

DPatrick
2017-09-27 19:37:14

Indeed, most students participated online. 4100 students total did Round 1, with 3600 of them taking the test online.

Indeed, most students participated online. 4100 students total did Round 1, with 3600 of them taking the test online.

DPatrick
2017-09-27 19:37:51

The mean score was 3.78 out of 10. A score of 8 out of 10 was sufficient to advance to Round 2, which will be in October.

The mean score was 3.78 out of 10. A score of 8 out of 10 was sufficient to advance to Round 2, which will be in October.

kenisaka
2017-09-27 19:38:03

may i still take the test or is it over?

may i still take the test or is it over?

DPatrick
2017-09-27 19:38:18

Round 1 has already concluded. We'll be going over the problems tonight.

Round 1 has already concluded. We'll be going over the problems tonight.

DPatrick
2017-09-27 19:38:40

You can visit the contest website at ams.org/wwtbam to learn more about the contest for next year.

You can visit the contest website at ams.org/wwtbam to learn more about the contest for next year.

Fortified
2017-09-27 19:38:53

How many people qualified for the second round???

How many people qualified for the second round???

DPatrick
2017-09-27 19:38:55

About 500.

About 500.

DPatrick
2017-09-27 19:39:07

We'll answer more questions at the end -- for now, let's move on to the problems!

We'll answer more questions at the end -- for now, let's move on to the problems!

DPatrick
2017-09-27 19:39:18

As I said, Round 1 consisted of 10 questions, with a 15-minute time limit.

As I said, Round 1 consisted of 10 questions, with a 15-minute time limit.

DPatrick
2017-09-27 19:39:26

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 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
2017-09-27 19:39:49

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

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

DPatrick
2017-09-27 19:40:29

Let's start at the beginning:

Let's start at the beginning:

DPatrick
2017-09-27 19:40:33

1. Find the $y$-intercept ($y$-coordinate only) of the line whose equation is $2x + 5y = 7$.

1. Find the $y$-intercept ($y$-coordinate only) of the line whose equation is $2x + 5y = 7$.

GeronimoStilton
2017-09-27 19:41:12

Plug in $x = 0$.

Plug in $x = 0$.

Designerd
2017-09-27 19:41:12

just test $x=0$

just test $x=0$

yeskay
2017-09-27 19:41:12

the y intercept is when x is zero

the y intercept is when x is zero

sgadekar
2017-09-27 19:41:12

plug in x=0

plug in x=0

fishy15
2017-09-27 19:41:16

this means x = 0, so sub in

this means x = 0, so sub in

DPatrick
2017-09-27 19:41:25

Right. All the points on the $y$-axis have in common that $x=0$.

Right. All the points on the $y$-axis have in common that $x=0$.

DPatrick
2017-09-27 19:41:31

So we can just plug in $x=0$ into our equation and solve for $y$.

So we can just plug in $x=0$ into our equation and solve for $y$.

seaskyline
2017-09-27 19:41:53

You can plug in 0 for x to get the equation 5y = 7 and divide by 5 to get y=7/5

You can plug in 0 for x to get the equation 5y = 7 and divide by 5 to get y=7/5

Liping
2017-09-27 19:41:53

THe y-intercept is the coordinate where x is 0. Then its 5y =7 and y=7/5

THe y-intercept is the coordinate where x is 0. Then its 5y =7 and y=7/5

pokemonduel
2017-09-27 19:41:59

and you get 7/5

and you get 7/5

DPatrick
2017-09-27 19:42:08

Exactly. This gives $5y = 7$, or $y = \boxed{\dfrac75}$.

Exactly. This gives $5y = 7$, or $y = \boxed{\dfrac75}$.

pianoman24
2017-09-27 19:42:14

That's like chapter Mathcounts Countdown round level

That's like chapter Mathcounts Countdown round level

DPatrick
2017-09-27 19:42:27

Indeed, this was the pretty straightforward warm-up problem.

Indeed, this was the pretty straightforward warm-up problem.

DPatrick
2017-09-27 19:42:37

Nearly 85% of participants got this correct.

Nearly 85% of participants got this correct.

DPatrick
2017-09-27 19:43:08

It gets harder almost right away. All of #2-#9 were solved by somewhere between 24% and 42% of students.

It gets harder almost right away. All of #2-#9 were solved by somewhere between 24% and 42% of students.

DPatrick
2017-09-27 19:43:15

2. $2 + 4 + 6 + 8 + \cdots + 2018 = \;?$

2. $2 + 4 + 6 + 8 + \cdots + 2018 = \;?$

mxzhang
2017-09-27 19:43:57

Arithmetic sequence!

Arithmetic sequence!

mxzhang
2017-09-27 19:43:57

There are $1009$ terms.

There are $1009$ terms.

rick101
2017-09-27 19:43:57

Pair up numbers: 2 and 2018, 4 and 2016, etc.

Pair up numbers: 2 and 2018, 4 and 2016, etc.

DPatrick
2017-09-27 19:44:05

Indeed, this is an

Indeed, this is an

**arithmetic series**: the difference between two consecutive summands is constant (in this case, 2).
DPatrick
2017-09-27 19:44:37

Its sum is equal to (the number of terms in the series) * (the average number in the series).

Its sum is equal to (the number of terms in the series) * (the average number in the series).

DPatrick
2017-09-27 19:45:08

Indeed, you can see this by pairing up terms: 2 and 2018, 4 and 2016, 6 and 2014, and so on. This gives pairs of terms each of whose average is the average of the entire series.

Indeed, you can see this by pairing up terms: 2 and 2018, 4 and 2016, 6 and 2014, and so on. This gives pairs of terms each of whose average is the average of the entire series.

yali306
2017-09-27 19:45:27

1009*1010

1009*1010

pianoman24
2017-09-27 19:45:27

Average number = first+last/2

Average number = first+last/2

DPatrick
2017-09-27 19:45:41

There are 1009 terms (the series is the sum of the first $2018/2 = 1009$ even positive integers.)

There are 1009 terms (the series is the sum of the first $2018/2 = 1009$ even positive integers.)

DPatrick
2017-09-27 19:45:58

And yes, the average term of an arithmetic sequence is just the average of the first and last term. So the average term is $(2+2018)/2 = 2020/2 = 1010$.

And yes, the average term of an arithmetic sequence is just the average of the first and last term. So the average term is $(2+2018)/2 = 2020/2 = 1010$.

prajna1225
2017-09-27 19:46:09

1009 . 1010

1009 . 1010

iks92
2017-09-27 19:46:09

1009*1010=1019090

1009*1010=1019090

Cidkip
2017-09-27 19:46:09

So the answer is $1009\cdot 1010 = 1009000 + 10090 = 1019090$

So the answer is $1009\cdot 1010 = 1009000 + 10090 = 1019090$

DPatrick
2017-09-27 19:46:15

Therefore, our sum is the product $(1009)(1010)$.

Therefore, our sum is the product $(1009)(1010)$.

DPatrick
2017-09-27 19:46:22

And to finish, $(1009)(1010) = 1010000 + 9090 = \boxed{1019090}$.

And to finish, $(1009)(1010) = 1010000 + 9090 = \boxed{1019090}$.

sde7
2017-09-27 19:46:30

factor out 2 and then sum of first 1009 numbers formula

factor out 2 and then sum of first 1009 numbers formula

neelbhalla
2017-09-27 19:46:36

divide the sequence by 2 giving you the sum of all numbers from 1...1009 which can be generalized by the formula n(n+1)/2

divide the sequence by 2 giving you the sum of all numbers from 1...1009 which can be generalized by the formula n(n+1)/2

DPatrick
2017-09-27 19:46:53

Definitely, as an alternative you could have gotten this by rewriting the given sum as $2(1+2+3+4+\cdots+1009)$, and then using the formula that the sum of the first $n$ positive integers is $\dfrac{n(n+1)}{2}$.

Definitely, as an alternative you could have gotten this by rewriting the given sum as $2(1+2+3+4+\cdots+1009)$, and then using the formula that the sum of the first $n$ positive integers is $\dfrac{n(n+1)}{2}$.

DPatrick
2017-09-27 19:47:37

I suspect that most of the wrong answers either were off by a factor of 2, or computed $1009 \cdot 1008$ instead.

I suspect that most of the wrong answers either were off by a factor of 2, or computed $1009 \cdot 1008$ instead.

DPatrick
2017-09-27 19:47:44

On to #3:

On to #3:

DPatrick
2017-09-27 19:47:47

3. $\left(\cos\dfrac{\pi}{12}+\sin\dfrac{\pi}{12}\right)\left(\cos\dfrac{\pi}{12}-\sin\dfrac{\pi}{12}\right) = \;?$

3. $\left(\cos\dfrac{\pi}{12}+\sin\dfrac{\pi}{12}\right)\left(\cos\dfrac{\pi}{12}-\sin\dfrac{\pi}{12}\right) = \;?$

DPatrick
2017-09-27 19:47:51

(We are instructed that no trig functions may appear in our final answer.)

(We are instructed that no trig functions may appear in our final answer.)

GeronimoStilton
2017-09-27 19:48:21

Use difference of squares first.

Use difference of squares first.

reddragon644
2017-09-27 19:48:21

Difference of squares?

Difference of squares?

pokemonduel
2017-09-27 19:48:21

its a difference of squares

its a difference of squares

DPatrick
2017-09-27 19:48:43

Do you know what $\cos\dfrac{\pi}{12}$ or $\sin\dfrac{\pi}{12}$ are? I don't off the top of my head. But we can simplify a bit first.

Do you know what $\cos\dfrac{\pi}{12}$ or $\sin\dfrac{\pi}{12}$ are? I don't off the top of my head. But we can simplify a bit first.

DPatrick
2017-09-27 19:49:00

It may be clearer to see what's going on if we set $c = \cos\dfrac{\pi}{12}$ and $s = \sin\dfrac{\pi}{12}$, so that our expression becomes $(c+s)(c-s)$. (This is often a good tactic: replace a complicated chunk in an expression with a variable, especially if it appears in multiple places.)

It may be clearer to see what's going on if we set $c = \cos\dfrac{\pi}{12}$ and $s = \sin\dfrac{\pi}{12}$, so that our expression becomes $(c+s)(c-s)$. (This is often a good tactic: replace a complicated chunk in an expression with a variable, especially if it appears in multiple places.)

Unlimited
2017-09-27 19:49:23

$c^2-s^2$!

$c^2-s^2$!

ConanC
2017-09-27 19:49:23

c^2-s^2

c^2-s^2

DPatrick
2017-09-27 19:49:37

And as many of you pointed out, this is the difference-of-squares factorization: $(c+s)(c-s) = c^2 - s^2$.

And as many of you pointed out, this is the difference-of-squares factorization: $(c+s)(c-s) = c^2 - s^2$.

DPatrick
2017-09-27 19:49:52

So our expression is equal to $\cos^2\dfrac{\pi}{12} - \sin^2\dfrac{\pi}{12}$.

So our expression is equal to $\cos^2\dfrac{\pi}{12} - \sin^2\dfrac{\pi}{12}$.

iks92
2017-09-27 19:50:14

double angle formula for cos

double angle formula for cos

Cidkip
2017-09-27 19:50:14

$\cos^2 \pi/12 - \sin^2 \pi/12,$ which looks a lot like the double angle identity...

$\cos^2 \pi/12 - \sin^2 \pi/12,$ which looks a lot like the double angle identity...

mxzhang
2017-09-27 19:50:14

The left hand side is $\cos^2\frac\pi{12}-\sin^2\frac\pi{12}=\cos^22\cdot\frac\pi{12}$.

The left hand side is $\cos^2\frac\pi{12}-\sin^2\frac\pi{12}=\cos^22\cdot\frac\pi{12}$.

vatatmaja
2017-09-27 19:50:14

this is just double angle for cosine

this is just double angle for cosine

stronto
2017-09-27 19:50:14

$ = \cos \frac{\pi}{6}$ by double-angle identities

$ = \cos \frac{\pi}{6}$ by double-angle identities

mxzhang
2017-09-27 19:50:14

Cosine double angle formula!

Cosine double angle formula!

sketchcomedyrules
2017-09-27 19:50:14

This is the cosine double angle formula!

This is the cosine double angle formula!

DPatrick
2017-09-27 19:50:22

Right! Now we can use the double-angle formula $\cos 2x = \cos^2x - \sin^2x$.

Right! Now we can use the double-angle formula $\cos 2x = \cos^2x - \sin^2x$.

DPatrick
2017-09-27 19:50:31

So our expression is just $\cos\dfrac{\pi}{6}$.

So our expression is just $\cos\dfrac{\pi}{6}$.

awesomemaths
2017-09-27 19:50:54

sqrt3/2

sqrt3/2

rapturt9
2017-09-27 19:50:59

$\sqrt{3}/2$

$\sqrt{3}/2$

DPatrick
2017-09-27 19:51:02

And that's $\boxed{\dfrac{\sqrt3}{2}}$.

And that's $\boxed{\dfrac{\sqrt3}{2}}$.

DPatrick
2017-09-27 19:51:30

This was actually the hardest out of the batch #2-#9: only 24% got this correct.

This was actually the hardest out of the batch #2-#9: only 24% got this correct.

DPatrick
2017-09-27 19:51:40

(#10 was a lot harder, though, as we'll see in a bit.)

(#10 was a lot harder, though, as we'll see in a bit.)

mikebreen
2017-09-27 19:51:44

That surprised us.

That surprised us.

DPatrick
2017-09-27 19:52:15

On to #4, which is one of the unique features of WWTBAM -- math history or trivia.

On to #4, which is one of the unique features of WWTBAM -- math history or trivia.

DPatrick
2017-09-27 19:52:19

4. What ancient Greek mathematician has a "sieve," used for finding prime numbers, named after him?

a. Archimedes b. Eratosthenes c. Euclid d. Pythagoras

4. What ancient Greek mathematician has a "sieve," used for finding prime numbers, named after him?

a. Archimedes b. Eratosthenes c. Euclid d. Pythagoras

OmicronGamma
2017-09-27 19:52:51

ERATOSTHENES' SIEVE

ERATOSTHENES' SIEVE

GeronimoStilton
2017-09-27 19:52:51

Sieve of Eratosthenes

Sieve of Eratosthenes

albertlu00
2017-09-27 19:52:51

sieve of eratosthenes

sieve of eratosthenes

pianoman24
2017-09-27 19:52:51

Sieve of Eratosthenes I coded that

Sieve of Eratosthenes I coded that

DPatrick
2017-09-27 19:52:56

It's $\boxed{\text{b. Eratosthenes}}$

It's $\boxed{\text{b. Eratosthenes}}$

DPatrick
2017-09-27 19:53:03

DPatrick
2017-09-27 19:53:12

*Image from http://www-history.mcs.st-andrews.ac.uk/Biographies/Eratosthenes.html*
DPatrick
2017-09-27 19:53:20

Eratosthenes lived 276-194 BC and was the chief librarian at the Library of Alexandria (in present-day Egypt, then part of the Greek empire).

Eratosthenes lived 276-194 BC and was the chief librarian at the Library of Alexandria (in present-day Egypt, then part of the Greek empire).

DPatrick
2017-09-27 19:53:30

The Sieve of Eratosthenes is a method for creating a list of prime numbers. A typical example is to find all the primes less than 100. We start with a chart from 2 to 100 (we don't include 1 because 1 is special -- it's neither prime nor composite):

The Sieve of Eratosthenes is a method for creating a list of prime numbers. A typical example is to find all the primes less than 100. We start with a chart from 2 to 100 (we don't include 1 because 1 is special -- it's neither prime nor composite):

DPatrick
2017-09-27 19:53:39

DPatrick
2017-09-27 19:53:58

Those of you who have seen it before: how does it work?

Those of you who have seen it before: how does it work?

neelbhalla
2017-09-27 19:54:32

take away all numbers divisble by 2, then take away all numbers divisible by 3 until you reach the max number(100)

take away all numbers divisble by 2, then take away all numbers divisible by 3 until you reach the max number(100)

pianoman24
2017-09-27 19:54:32

2 is the first prime, so now all the multiples of 2 can be erased since they are not primes

2 is the first prime, so now all the multiples of 2 can be erased since they are not primes

Cidkip
2017-09-27 19:54:32

Start with 2. Circle it, then cross off all multiples of 2

Start with 2. Circle it, then cross off all multiples of 2

DPatrick
2017-09-27 19:54:38

Right. The smallest number in the chart -- 2 -- is prime. And any multiple of 2 is not prime.

Right. The smallest number in the chart -- 2 -- is prime. And any multiple of 2 is not prime.

DPatrick
2017-09-27 19:54:44

So we circle 2, and we cross out all the numbers that are multiples of 2.

So we circle 2, and we cross out all the numbers that are multiples of 2.

DPatrick
2017-09-27 19:54:47

Generic_Username
2017-09-27 19:55:00

go to the first unmarked number and call it prime, mark all multiples, repeat

go to the first unmarked number and call it prime, mark all multiples, repeat

bubblestick3
2017-09-27 19:55:00

first remove the multiples of 2, then 3, then 5, then keep doing it with other primes

first remove the multiples of 2, then 3, then 5, then keep doing it with other primes

awesomemaths
2017-09-27 19:55:02

then 3

then 3

DPatrick
2017-09-27 19:55:16

Right. Now the smallest unmarked number is 3. It's prime.

Right. Now the smallest unmarked number is 3. It's prime.

DPatrick
2017-09-27 19:55:20

So we circle it and cross out multiples of 3:

So we circle it and cross out multiples of 3:

DPatrick
2017-09-27 19:55:25

prajna1225
2017-09-27 19:55:34

then 5

then 5

Unlimited
2017-09-27 19:55:34

Yup and we keep doing that

Yup and we keep doing that

DPatrick
2017-09-27 19:55:39

We can continue in this way: we circle the smallest remaining unmarked number (which must be prime, as it's not a multiple of a smaller number), and cross out its multiples.

We can continue in this way: we circle the smallest remaining unmarked number (which must be prime, as it's not a multiple of a smaller number), and cross out its multiples.

DPatrick
2017-09-27 19:55:48

So we next do this for 5:

So we next do this for 5:

DPatrick
2017-09-27 19:55:54

DPatrick
2017-09-27 19:56:00

And then we do it for 7:

And then we do it for 7:

DPatrick
2017-09-27 19:56:04

pokemonduel
2017-09-27 19:56:11

just keep crossing out multiples of primes till you reach 100 and you dont have to go more than 10

just keep crossing out multiples of primes till you reach 100 and you dont have to go more than 10

Unlimited
2017-09-27 19:56:11

Until 10

Until 10

DPatrick
2017-09-27 19:56:26

Nothing more need be done. Any number less than 100 that's composite must be a multiple of 2, 3, 5, or 7, because those are all the primes less than $\sqrt{100} = 10$. So all the numbers that remain uncrossed are prime. (I won't bother circling them.)

Nothing more need be done. Any number less than 100 that's composite must be a multiple of 2, 3, 5, or 7, because those are all the primes less than $\sqrt{100} = 10$. So all the numbers that remain uncrossed are prime. (I won't bother circling them.)

DPatrick
2017-09-27 19:56:41

So we see that the primes less than 100 are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97.

So we see that the primes less than 100 are: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97.

DPatrick
2017-09-27 19:57:10

Eratosthenes is also particularly known for a couple of other insights. First, he was one of the first people known to attempt to accurately measure the circumference of the Earth. He did this by comparing the noon shadow at two different locations and using the difference in their angles. There is some scholarly dispute over the units that he used, but his calculation is believed to be within 10-15% of the actual circumference of the Earth.

Eratosthenes is also particularly known for a couple of other insights. First, he was one of the first people known to attempt to accurately measure the circumference of the Earth. He did this by comparing the noon shadow at two different locations and using the difference in their angles. There is some scholarly dispute over the units that he used, but his calculation is believed to be within 10-15% of the actual circumference of the Earth.

DPatrick
2017-09-27 19:57:30

That's pretty impressive for over 2000 years ago.

That's pretty impressive for over 2000 years ago.

DPatrick
2017-09-27 19:57:39

Second, Eratosthenes was one of the first people to recognize the need for leap years. He was able to determine that the length of a year was approximately 365.25 days, so that every 4th year we would need a leap day in the calendar.

Second, Eratosthenes was one of the first people to recognize the need for leap years. He was able to determine that the length of a year was approximately 365.25 days, so that every 4th year we would need a leap day in the calendar.

DPatrick
2017-09-27 19:57:59

Next:

Next:

DPatrick
2017-09-27 19:58:00

5. Write $\dfrac{\log_5 9}{\log_{25} 3}$ as a rational number.

5. Write $\dfrac{\log_5 9}{\log_{25} 3}$ as a rational number.

Pi_3.14_Squared
2017-09-27 19:58:22

Use change of base.

Use change of base.

awang11
2017-09-27 19:58:22

Use log properties

Use log properties

rick101
2017-09-27 19:58:22

Use change of base formula

Use change of base formula

DPatrick
2017-09-27 19:58:36

You might know some log identities that can help you out, but they're easy to mess up if you're not careful.

You might know some log identities that can help you out, but they're easy to mess up if you're not careful.

DPatrick
2017-09-27 19:58:49

I prefer to introduce variables because it's way easier (for me) to keep track.

I prefer to introduce variables because it's way easier (for me) to keep track.

reddragon644
2017-09-27 19:58:57

5 is the base and 9 is the answer to the top so do $5^x=9$

5 is the base and 9 is the answer to the top so do $5^x=9$

DPatrick
2017-09-27 19:59:07

Exactly. Let's set $x = \log_5 9$ and $y = \log_{25} 3$, so that the quantity we want to find is $\dfrac{x}{y}$.

Exactly. Let's set $x = \log_5 9$ and $y = \log_{25} 3$, so that the quantity we want to find is $\dfrac{x}{y}$.

DPatrick
2017-09-27 19:59:27

So then we have $5^x = 9$ and $25^y = 3$ by the definition of log.

So then we have $5^x = 9$ and $25^y = 3$ by the definition of log.

DarkRunner
2017-09-27 20:00:07

Oh, and rewrite 25 as 5^

Oh, and rewrite 25 as 5^

DarkRunner
2017-09-27 20:00:07

5^2*

5^2*

DPatrick
2017-09-27 20:00:19

Yes, good idea: they're easier to compare if they have the same base.

Yes, good idea: they're easier to compare if they have the same base.

DPatrick
2017-09-27 20:00:31

So let's write the second one as $(5^2)^y = 3$.

So let's write the second one as $(5^2)^y = 3$.

DPatrick
2017-09-27 20:00:35

This simplifies to $5^{2y} = 3$.

This simplifies to $5^{2y} = 3$.

DPatrick
2017-09-27 20:00:50

So now we have $5^x = 9$ and $5^{2y} = 3$.

So now we have $5^x = 9$ and $5^{2y} = 3$.

neelbhalla
2017-09-27 20:01:06

square 5^2y so they are both 9

square 5^2y so they are both 9

DPatrick
2017-09-27 20:01:13

Good idea too.

Good idea too.

DPatrick
2017-09-27 20:01:19

If we square it, we get $(5^{2y})^2 = 9$. This simplifies to $5^{4y} = 9$.

If we square it, we get $(5^{2y})^2 = 9$. This simplifies to $5^{4y} = 9$.

rick101
2017-09-27 20:01:32

x=4y

x=4y

sgadekar
2017-09-27 20:01:32

then x=4y

then x=4y

DPatrick
2017-09-27 20:01:50

Right. We have $5^x = 9$ and $5^{4y} = 9$. So $x = 4y$.

Right. We have $5^x = 9$ and $5^{4y} = 9$. So $x = 4y$.

ThorJames
2017-09-27 20:02:00

So the answer is $4$

So the answer is $4$

kz1356
2017-09-27 20:02:00

x/y=4

x/y=4

prajna1225
2017-09-27 20:02:00

x=4y, ans is 4y/y=4

x=4y, ans is 4y/y=4

DPatrick
2017-09-27 20:02:07

And to finish, $\dfrac{x}{y} = \boxed{4}$ is our answer.

And to finish, $\dfrac{x}{y} = \boxed{4}$ is our answer.

mikebreen
2017-09-27 20:02:28

Nice solution.

Nice solution.

DPatrick
2017-09-27 20:02:46

Like I said, you can certainly do this more slickly with log-identities. But this way I'm pretty sure I didn't mess anything up!

Like I said, you can certainly do this more slickly with log-identities. But this way I'm pretty sure I didn't mess anything up!

DPatrick
2017-09-27 20:03:02

6. For which one of the following choices for $m$ are the base $m$ numbers $25_m$ and $27_m$ both prime?

a. 8 b. 9 c. 11 d. 12 e. 13

6. For which one of the following choices for $m$ are the base $m$ numbers $25_m$ and $27_m$ both prime?

a. 8 b. 9 c. 11 d. 12 e. 13

DPatrick
2017-09-27 20:03:36

We could brute-force check them all, but what can we notice that may make things easier?

We could brute-force check them all, but what can we notice that may make things easier?

ThorJames
2017-09-27 20:03:50

write into 2m+5 and 2m+7

write into 2m+5 and 2m+7

reddragon644
2017-09-27 20:03:50

you put the number in terms of m

you put the number in terms of m

pianoman24
2017-09-27 20:03:50

Rewrite them as 2m+5 and 2m+7 and test

Rewrite them as 2m+5 and 2m+7 and test

Generic_Username
2017-09-27 20:03:50

they are twin primes

they are twin primes

shootingstar8
2017-09-27 20:03:50

they are twin primes

they are twin primes

DPatrick
2017-09-27 20:03:58

Right: we need to find which $m$ makes $2m+5$ and $2m+7$ both prime.

Right: we need to find which $m$ makes $2m+5$ and $2m+7$ both prime.

DPatrick
2017-09-27 20:04:11

And it's clear that these are consecutive odd numbers.

And it's clear that these are consecutive odd numbers.

DPatrick
2017-09-27 20:04:26

If they're both prime, what does that say about $2m + 3$?

If they're both prime, what does that say about $2m + 3$?

pianoman24
2017-09-27 20:04:43

It isn't prime

It isn't prime

reddragon644
2017-09-27 20:04:43

it is not prime

it is not prime

DPatrick
2017-09-27 20:04:52

Indeed, but more specifically...?

Indeed, but more specifically...?

awesomemaths
2017-09-27 20:04:58

it has to be divisble by 3

it has to be divisble by 3

DPatrick
2017-09-27 20:05:19

Right. If we've got three consecutive prime numbers, and two of them are prime, then the other one has to be a multiple of 3.

Right. If we've got three consecutive prime numbers, and two of them are prime, then the other one has to be a multiple of 3.

DPatrick
2017-09-27 20:05:30

So $2m + 3$ must be a multiple of 3. What does that say about $m$?

So $2m + 3$ must be a multiple of 3. What does that say about $m$?

GeronimoStilton
2017-09-27 20:05:53

It's a multiple of $3$!

It's a multiple of $3$!

Mudkipswims42
2017-09-27 20:05:53

multiple of 3

multiple of 3

math-family
2017-09-27 20:05:53

It has to be a multiple of 3

It has to be a multiple of 3

ShreyJ
2017-09-27 20:05:53

it is divisible by 3

it is divisible by 3

yali306
2017-09-27 20:05:53

It's a multiple of 3

It's a multiple of 3

DPatrick
2017-09-27 20:06:09

Right. For $2m+3$ to be a multiple of 3, $m$ must be a multiple of 3 too.

Right. For $2m+3$ to be a multiple of 3, $m$ must be a multiple of 3 too.

pianoman24
2017-09-27 20:06:15

We only have to check 9 or 12

We only have to check 9 or 12

rick101
2017-09-27 20:06:15

We've eliminated 3 choices.

We've eliminated 3 choices.

DPatrick
2017-09-27 20:06:24

Right: the answer must be (b) 9 or (d) 12.

Right: the answer must be (b) 9 or (d) 12.

DPatrick
2017-09-27 20:06:37

And at this point we can just check the two possibilities.

And at this point we can just check the two possibilities.

pianoman24
2017-09-27 20:06:58

9 gives 23 and 25, and 25 isn't prime, so it has to be 12 (we only even have to check 1 by process of elimination!)

9 gives 23 and 25, and 25 isn't prime, so it has to be 12 (we only even have to check 1 by process of elimination!)

DPatrick
2017-09-27 20:07:09

Indeed, $m=9$ doesn't work because $27_9 = 25$ isn't prime.

Indeed, $m=9$ doesn't work because $27_9 = 25$ isn't prime.

DPatrick
2017-09-27 20:07:25

So by process of elimination the answer must be $\boxed{\text{(d) } 12}$.

So by process of elimination the answer must be $\boxed{\text{(d) } 12}$.

DPatrick
2017-09-27 20:07:35

And indeed, $25_{12} = 29$ and $27_{12} = 31$ are both prime.

And indeed, $25_{12} = 29$ and $27_{12} = 31$ are both prime.

DPatrick
2017-09-27 20:08:00

Next up:

Next up:

DPatrick
2017-09-27 20:08:04

7. You and two friends decide to pay a restaurant bill by having each person flip a fair, two-sided coin. If all three coins show heads or all show tails, you will split the bill three ways. Otherwise, the person whose coin landed differently from the other two will pay the entire bill. What is the probability that you will not have to pay anything?

7. You and two friends decide to pay a restaurant bill by having each person flip a fair, two-sided coin. If all three coins show heads or all show tails, you will split the bill three ways. Otherwise, the person whose coin landed differently from the other two will pay the entire bill. What is the probability that you will not have to pay anything?

albertlu00
2017-09-27 20:08:29

casework

casework

Pi_3.14_Squared
2017-09-27 20:08:29

Casework?

Casework?

DPatrick
2017-09-27 20:08:59

Right. There are two cases listed in the problem: we all flip the same, or someone flips different.

Right. There are two cases listed in the problem: we all flip the same, or someone flips different.

DPatrick
2017-09-27 20:09:46

But if we all flip the same, I'll definitely have to pay. So we can ignore that case, because it's not part of what we want!

But if we all flip the same, I'll definitely have to pay. So we can ignore that case, because it's not part of what we want!

DPatrick
2017-09-27 20:10:06

What's the probability that we don't all flip the same?

What's the probability that we don't all flip the same?

IlinoisMathlete
2017-09-27 20:10:23

3/4

3/4

TopNotchMath
2017-09-27 20:10:23

3/4

3/4

awang11
2017-09-27 20:10:23

3/4

3/4

Plasma_Vortex
2017-09-27 20:10:23

3/4

3/4

NLMath
2017-09-27 20:10:23

3/4

3/4

DPatrick
2017-09-27 20:10:43

Right. There are $2^3 = 8$ possible outcomes for the flips, and only 2 of those (HHH and TTT) are where we all flip the same.

Right. There are $2^3 = 8$ possible outcomes for the flips, and only 2 of those (HHH and TTT) are where we all flip the same.

DPatrick
2017-09-27 20:11:01

So the other 6 outcomes, we don't all flip the same, and that occurs with probability $\dfrac68 = \dfrac34$.

So the other 6 outcomes, we don't all flip the same, and that occurs with probability $\dfrac68 = \dfrac34$.

DPatrick
2017-09-27 20:11:25

And if we don't flip the same, what's the probably I don't have to pay?

And if we don't flip the same, what's the probably I don't have to pay?

Hippoman
2017-09-27 20:11:49

2/3

2/3

GeronimoStilton
2017-09-27 20:11:49

$\frac{2}{3}$

$\frac{2}{3}$

awang11
2017-09-27 20:11:49

2/3

2/3

DPatrick
2017-09-27 20:12:07

Right. By symmetry, each of the 3 of us are equally likely to be the person who has to pay it all.

Right. By symmetry, each of the 3 of us are equally likely to be the person who has to pay it all.

DPatrick
2017-09-27 20:12:24

So I have probability 1/3 that I'll have to pay it all, leaving probability $\dfrac23$ that I'll have to pay nothing.

So I have probability 1/3 that I'll have to pay it all, leaving probability $\dfrac23$ that I'll have to pay nothing.

awang11
2017-09-27 20:12:34

Then 3/4 * 2/3 = $\boxed{1/2}$

Then 3/4 * 2/3 = $\boxed{1/2}$

DPatrick
2017-09-27 20:13:18

Right: to summarize and finish, there's a $\dfrac34$ probability that one person will end up paying it all, and in that case there's a $\dfrac23$ probability that person won't be me.

Right: to summarize and finish, there's a $\dfrac34$ probability that one person will end up paying it all, and in that case there's a $\dfrac23$ probability that person won't be me.

DPatrick
2017-09-27 20:13:38

So I get a free lunch with probability $\dfrac34 \cdot \dfrac23 = \dfrac24 = \boxed{\dfrac12}$.

So I get a free lunch with probability $\dfrac34 \cdot \dfrac23 = \dfrac24 = \boxed{\dfrac12}$.

DPatrick
2017-09-27 20:14:15

As with most probability problems, there were certainly other ways you could have computed this.

As with most probability problems, there were certainly other ways you could have computed this.

DPatrick
2017-09-27 20:14:46

I'll leave that for you to think about on your own.

I'll leave that for you to think about on your own.

DPatrick
2017-09-27 20:14:53

Let's move on -- just 3 problems to go!

Let's move on -- just 3 problems to go!

DPatrick
2017-09-27 20:14:57

8. Suppose that $m$ and $n$ are positive integers such that $5m + 3n = 41$. What is the smallest possible value for $|m^2 - n^2|$?

8. Suppose that $m$ and $n$ are positive integers such that $5m + 3n = 41$. What is the smallest possible value for $|m^2 - n^2|$?

DaniyalQazi2
2017-09-27 20:15:22

Just bash it out. It's only positive integers.

Just bash it out. It's only positive integers.

DPatrick
2017-09-27 20:15:35

Indeed, there can't possibly be very many solutions in positive integers to this equation. Maybe we can just list them all.

Indeed, there can't possibly be very many solutions in positive integers to this equation. Maybe we can just list them all.

DPatrick
2017-09-27 20:16:12

Is there an easy way to list all the solutions using positive integers?

Is there an easy way to list all the solutions using positive integers?

DaniyalQazi2
2017-09-27 20:16:34

The only possible pairs are (m,n) - (1,12), (4,7), (7,2)

The only possible pairs are (m,n) - (1,12), (4,7), (7,2)

pianoman24
2017-09-27 20:16:34

1 and 12, 4 and 7, 7 and 2 are our choices

1 and 12, 4 and 7, 7 and 2 are our choices

Pi_3.14_Squared
2017-09-27 20:16:34

You only have to test the pairs (1,12) (4,7) (7,2)

You only have to test the pairs (1,12) (4,7) (7,2)

IlinoisMathlete
2017-09-27 20:16:40

go down by multiples of 5 and see if the number is divisible by 3

go down by multiples of 5 and see if the number is divisible by 3

randomsolver
2017-09-27 20:17:01

(m,n) = (1,12), (4,7), (7,2)

(m,n) = (1,12), (4,7), (7,2)

losedude
2017-09-27 20:17:01

because 5m has to end in a 5 or 0, 3m must end in 1 or 6

because 5m has to end in a 5 or 0, 3m must end in 1 or 6

DPatrick
2017-09-27 20:17:21

Indeed, there are lots of ways to find the list of solutions, including just plain old brute force.

Indeed, there are lots of ways to find the list of solutions, including just plain old brute force.

DPatrick
2017-09-27 20:17:46

What I did is essentially was losedude suggested: notice that $3n$ has to be one more than a multiple of 5.

What I did is essentially was losedude suggested: notice that $3n$ has to be one more than a multiple of 5.

DPatrick
2017-09-27 20:17:56

This means $n$ must be 2 more than a multiple of 5.

This means $n$ must be 2 more than a multiple of 5.

DPatrick
2017-09-27 20:18:13

(You can experiment and check this if you're not sure about my last assertion.)

(You can experiment and check this if you're not sure about my last assertion.)

DPatrick
2017-09-27 20:18:19

So we must have $n = 2, 7, \text{ or } 12$.

So we must have $n = 2, 7, \text{ or } 12$.

DPatrick
2017-09-27 20:18:28

And now we can just make a little chart.

And now we can just make a little chart.

DPatrick
2017-09-27 20:18:31

\[

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

m & n & m^2 - n^2 \\ \hline

7 & 2 & 45 \\

4 & 7 & -33 \\

1 & 12 & -143

\end{array}

\]

\[

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

m & n & m^2 - n^2 \\ \hline

7 & 2 & 45 \\

4 & 7 & -33 \\

1 & 12 & -143

\end{array}

\]

prajna1225
2017-09-27 20:18:42

we must make m close to n

we must make m close to n

bubblestick3
2017-09-27 20:18:42

but m and n have to be as close as possible, so its 7^2 - 4^2 = 33

but m and n have to be as close as possible, so its 7^2 - 4^2 = 33

DPatrick
2017-09-27 20:18:55

Right: we see that the smallest value of $|m^2 - n^2|$ is the middle row, $\boxed{33}$.

Right: we see that the smallest value of $|m^2 - n^2|$ is the middle row, $\boxed{33}$.

DPatrick
2017-09-27 20:19:20

On to geometry:

On to geometry:

DPatrick
2017-09-27 20:19:24

9. In rectangle $ABCD$ below, $\overline{DP}$ is perpendicular to $\overline{PC}$. Find $x$, the distance from $A$ to $P$, where $P$ is between $A$ and the midpoint of segment $\overline{AB}$.

a. $3-2\sqrt2$ b. $3-\sqrt2$ c. $3-\sqrt7$ d. $3-\sqrt7/2$

9. In rectangle $ABCD$ below, $\overline{DP}$ is perpendicular to $\overline{PC}$. Find $x$, the distance from $A$ to $P$, where $P$ is between $A$ and the midpoint of segment $\overline{AB}$.

a. $3-2\sqrt2$ b. $3-\sqrt2$ c. $3-\sqrt7$ d. $3-\sqrt7/2$

DPatrick
2017-09-27 20:19:31

DPatrick
2017-09-27 20:19:36

Note that the provided picture is clearly not to scale: there's no way that $CD = 6AD$ in the drawing. So we need to be careful about "assuming" facts from the picture.

Note that the provided picture is clearly not to scale: there's no way that $CD = 6AD$ in the drawing. So we need to be careful about "assuming" facts from the picture.

Pi_3.14_Squared
2017-09-27 20:20:09

Let AP=x, then PB=6-x

Let AP=x, then PB=6-x

DPatrick
2017-09-27 20:20:23

Let's label $x = AP$ in the drawing (what we're trying to find), and add the other missing segments:

Let's label $x = AP$ in the drawing (what we're trying to find), and add the other missing segments:

DPatrick
2017-09-27 20:20:28

CforSeahorse
2017-09-27 20:20:34

Similar Triangles

Similar Triangles

tworigami
2017-09-27 20:20:34

we use similar triangles

we use similar triangles

GeronimoStilton
2017-09-27 20:20:34

Let's use similar triangles!

Let's use similar triangles!

DPatrick
2017-09-27 20:20:40

We have similar triangles!

We have similar triangles!

DPatrick
2017-09-27 20:20:52

To see this, note that since $\angle DPC$ is right, the angles $\angle APD$ and $\angle BPC$ must sum to $90^\circ$.

To see this, note that since $\angle DPC$ is right, the angles $\angle APD$ and $\angle BPC$ must sum to $90^\circ$.

DPatrick
2017-09-27 20:20:57

DPatrick
2017-09-27 20:21:11

But this means that $\angle ADP$ is the red angle, and $\angle BCP$ is the blue angle, since the two acute angles in a right triangle must sum to $90^\circ$.

But this means that $\angle ADP$ is the red angle, and $\angle BCP$ is the blue angle, since the two acute angles in a right triangle must sum to $90^\circ$.

DPatrick
2017-09-27 20:21:14

DPatrick
2017-09-27 20:21:27

In particular, $\triangle APD \sim \triangle BCP$.

In particular, $\triangle APD \sim \triangle BCP$.

ThorJames
2017-09-27 20:21:52

6-x=1/x

6-x=1/x

pianoman24
2017-09-27 20:21:52

(6-x)/1 = 1/x

(6-x)/1 = 1/x

IlinoisMathlete
2017-09-27 20:22:01

1/x=6-x/1?

1/x=6-x/1?

DPatrick
2017-09-27 20:22:05

So we have the proportion $\dfrac{x}{1} = \dfrac{1}{6-x}$.

So we have the proportion $\dfrac{x}{1} = \dfrac{1}{6-x}$.

willmathxu
2017-09-27 20:22:38

6x-x^2=1

6x-x^2=1

pianoman24
2017-09-27 20:22:38

6x-x^2=1 (cross multiply)

6x-x^2=1 (cross multiply)

yali306
2017-09-27 20:22:38

6x - x^2 = 1

6x - x^2 = 1

reddragon644
2017-09-27 20:22:38

quadratics!!!

quadratics!!!

DPatrick
2017-09-27 20:22:48

Right, this simplifies to $x(6-x) = 1$, or $x^2 - 6x + 1 = 0$.

Right, this simplifies to $x(6-x) = 1$, or $x^2 - 6x + 1 = 0$.

DPatrick
2017-09-27 20:23:00

And the solutions to this are $x = \dfrac{6 \pm \sqrt{36 - 4}}{2} = 3 \pm 2\sqrt2$.

And the solutions to this are $x = \dfrac{6 \pm \sqrt{36 - 4}}{2} = 3 \pm 2\sqrt2$.

brainiac1
2017-09-27 20:23:32

Obviously the solution less than 3 must be the correct one

Obviously the solution less than 3 must be the correct one

CforSeahorse
2017-09-27 20:23:32

3 + 2sqrt2 is too big

3 + 2sqrt2 is too big

DPatrick
2017-09-27 20:23:49

Clearly only solution $\boxed{\text{a. } 3-2\sqrt2}$ works. $3 + 2\sqrt2$ isn't even a choice.

Clearly only solution $\boxed{\text{a. } 3-2\sqrt2}$ works. $3 + 2\sqrt2$ isn't even a choice.

DPatrick
2017-09-27 20:23:58

And we're told $P$ is closer to $A$ than to $B$, so $x < 3$.

And we're told $P$ is closer to $A$ than to $B$, so $x < 3$.

mikebreen
2017-09-27 20:23:58

People did much better on this problem than I would have in high school. Good for them/you!

People did much better on this problem than I would have in high school. Good for them/you!

DPatrick
2017-09-27 20:24:36

OK, as I said, numbers 2-9 were roughly equal in difficulty. Each was solved by somewhere between 24% and 42% of participants.

OK, as I said, numbers 2-9 were roughly equal in difficulty. Each was solved by somewhere between 24% and 42% of participants.

DPatrick
2017-09-27 20:24:49

Not so #10. Only about 7% got this one. Here we go...

Not so #10. Only about 7% got this one. Here we go...

DPatrick
2017-09-27 20:24:56

10. What is the largest prime factor of $2^{14} + 1$.

10. What is the largest prime factor of $2^{14} + 1$.

DPatrick
2017-09-27 20:25:40

Some of you who are very experienced in algebra may have recognized what to do right away. But let's see how we can stumble our way there.

Some of you who are very experienced in algebra may have recognized what to do right away. But let's see how we can stumble our way there.

DPatrick
2017-09-27 20:26:00

Maybe a lot of you tried brute force first -- I did.

Maybe a lot of you tried brute force first -- I did.

DPatrick
2017-09-27 20:26:10

You may know that $2^{10} = 1024$. (This is a really useful fact to memorize.)

You may know that $2^{10} = 1024$. (This is a really useful fact to memorize.)

DPatrick
2017-09-27 20:26:20

So $2^{14} = 2^{10} \cdot 2^4 = 1024 \cdot 16 = 16384$.

So $2^{14} = 2^{10} \cdot 2^4 = 1024 \cdot 16 = 16384$.

Pi_3.14_Squared
2017-09-27 20:26:34

now add one

now add one

DPatrick
2017-09-27 20:26:41

Right. So the number we are given is $16385$.

Right. So the number we are given is $16385$.

DPatrick
2017-09-27 20:26:49

Clearly 5 is a prime factor, and we can divide to get $16385 = 5 \cdot 3277$.

Clearly 5 is a prime factor, and we can divide to get $16385 = 5 \cdot 3277$.

DPatrick
2017-09-27 20:27:12

But now we're in a bit of a pickle. Is 3277 prime? If not, how does it factor?

But now we're in a bit of a pickle. Is 3277 prime? If not, how does it factor?

DPatrick
2017-09-27 20:27:21

It's really hard to tell, and there's no clever way to check. This looks like a lot of brute force.

It's really hard to tell, and there's no clever way to check. This looks like a lot of brute force.

DPatrick
2017-09-27 20:27:35

It doable, but maybe we should back up a step -- it feels like we lost some information when we replaced $2^{14} + 1$ with $16385$.

It doable, but maybe we should back up a step -- it feels like we lost some information when we replaced $2^{14} + 1$ with $16385$.

DPatrick
2017-09-27 20:27:49

What could we do algebraically with $2^{14}+1$? It is a sum of two perfect squares -- does that help?

What could we do algebraically with $2^{14}+1$? It is a sum of two perfect squares -- does that help?

reddragon644
2017-09-27 20:28:00

$(2^7+1)^2-2^8$

$(2^7+1)^2-2^8$

DPatrick
2017-09-27 20:28:14

Indeed, there's always the identity $x^2 + y^2 = (x+y)^2 - 2xy$.

Indeed, there's always the identity $x^2 + y^2 = (x+y)^2 - 2xy$.

DPatrick
2017-09-27 20:28:21

In this case, that gives us $2^{14} + 1 = (2^7+1)^2 - 2 \cdot 2^7$.

In this case, that gives us $2^{14} + 1 = (2^7+1)^2 - 2 \cdot 2^7$.

DPatrick
2017-09-27 20:28:51

Or in other words, it's $(2^7+1)^2 - 2^8$.

Or in other words, it's $(2^7+1)^2 - 2^8$.

reddragon644
2017-09-27 20:28:58

use differences of squares

use differences of squares

Pi_3.14_Squared
2017-09-27 20:28:58

And this equals (2^7+1+2^4)(2^7+1-2^4)

And this equals (2^7+1+2^4)(2^7+1-2^4)

GeronimoStilton
2017-09-27 20:28:58

Now we can factor!

Now we can factor!

SirCalcsALot
2017-09-27 20:28:58

You can factor this with difference of squares!

You can factor this with difference of squares!

CforSeahorse
2017-09-27 20:28:58

2^8 is a perfect square

2^8 is a perfect square

DPatrick
2017-09-27 20:29:05

Aha! It's a difference of two perfect squares!

Aha! It's a difference of two perfect squares!

DPatrick
2017-09-27 20:29:13

So our number factors as $(2^7+1+2^4)(2^7+1-2^4)$.

So our number factors as $(2^7+1+2^4)(2^7+1-2^4)$.

DPatrick
2017-09-27 20:29:24

This is $145 \cdot 113$.

This is $145 \cdot 113$.

DPatrick
2017-09-27 20:29:34

So after factoring out the 5 as before, we get $5 \cdot 29 \cdot 113$.

So after factoring out the 5 as before, we get $5 \cdot 29 \cdot 113$.

pianoman24
2017-09-27 20:29:41

113 is prime

113 is prime

reddragon644
2017-09-27 20:29:41

so 113

so 113

mikebreen
2017-09-27 20:29:45

This is how we did it.

This is how we did it.

Pi_3.14_Squared
2017-09-27 20:29:47

113 is prime.

113 is prime.

andrusha2
2017-09-27 20:29:47

$113$ is our answer!

$113$ is our answer!

yali306
2017-09-27 20:29:47

Is 113 prime...

Is 113 prime...

DPatrick
2017-09-27 20:29:54

Yes, 113 is prime! (This is easy to check: since $11^2 = 121 > 113$, we only need to check divisibility by 2, 3, 5, and 7, and none of them work.)

Yes, 113 is prime! (This is easy to check: since $11^2 = 121 > 113$, we only need to check divisibility by 2, 3, 5, and 7, and none of them work.)

DPatrick
2017-09-27 20:30:01

So we have the prime factorization $2^{14} + 1 = 5 \cdot 29 \cdot 113$, and thus $\boxed{113}$ is the largest prime factor.

So we have the prime factorization $2^{14} + 1 = 5 \cdot 29 \cdot 113$, and thus $\boxed{113}$ is the largest prime factor.

Pi_3.14_Squared
2017-09-27 20:30:21

Sophie Germain?

Sophie Germain?

DaniyalQazi2
2017-09-27 20:30:21

Sophie Germaine Identity

Sophie Germaine Identity

ShreyJ
2017-09-27 20:30:21

We could use Sophie-Germain

We could use Sophie-Germain

CforSeahorse
2017-09-27 20:30:21

Could you not also use Sophie Germain?

Could you not also use Sophie Germain?

DPatrick
2017-09-27 20:30:32

Indeed, what we did was just a special case of the

\[ 4x^4 + y^4 = (2x^2+y^2)^2 - 4x^2y^2 = (2x^2+y^2+2xy)(2x^2+y^2-2xy).\]

Indeed, what we did was just a special case of the

**Sophie Germain Identity**, which is\[ 4x^4 + y^4 = (2x^2+y^2)^2 - 4x^2y^2 = (2x^2+y^2+2xy)(2x^2+y^2-2xy).\]

DPatrick
2017-09-27 20:30:46

In our problem, $x = 2^3$ and $y = 1$ gives the above factorization.

In our problem, $x = 2^3$ and $y = 1$ gives the above factorization.

DPatrick
2017-09-27 20:31:22

So that's it! On the actual contest: if you got 8 or more correct, congratulations! -- you're moving on to Round 2. Your teacher should have your invitation (or should get it soon). Round 2 will be held October 9-25. And come back here for our Round 2 Math Jam on Thursday, October 26 at 7:30 pm ET / 4:30 pm PT.

So that's it! On the actual contest: if you got 8 or more correct, congratulations! -- you're moving on to Round 2. Your teacher should have your invitation (or should get it soon). Round 2 will be held October 9-25. And come back here for our Round 2 Math Jam on Thursday, October 26 at 7:30 pm ET / 4:30 pm PT.

DPatrick
2017-09-27 20:31:37

After Round 2 is complete, 12 students will be invited to compete in the National Finals, live in San Diego at the 2018 Joint Mathematics Meetings on January 13. Travel costs to and from San Diego will be covered by the AMS. This is especially exciting for us this year, as San Diego is also the hometown of AoPS, so we expect that many AoPS staff will be in attendance at the Finals!

After Round 2 is complete, 12 students will be invited to compete in the National Finals, live in San Diego at the 2018 Joint Mathematics Meetings on January 13. Travel costs to and from San Diego will be covered by the AMS. This is especially exciting for us this year, as San Diego is also the hometown of AoPS, so we expect that many AoPS staff will be in attendance at the Finals!

DPatrick
2017-09-27 20:31:53

Here's how the 12 finalists will be determined: 10 of the 12 will be the top scorer from Round 2 in each of the following regions:

Here's how the 12 finalists will be determined: 10 of the 12 will be the top scorer from Round 2 in each of the following regions:

DPatrick
2017-09-27 20:31:57

DPatrick
2017-09-27 20:32:07

The other two contestants will be the top scorer in the United Kingdom and the top scorer in the San Diego metro area (or, I suppose, the second-highest scorer, if the highest scorer in Region 9 happens to be from San Diego).

The other two contestants will be the top scorer in the United Kingdom and the top scorer in the San Diego metro area (or, I suppose, the second-highest scorer, if the highest scorer in Region 9 happens to be from San Diego).

DPatrick
2017-09-27 20:32:17

The National Finals are held live in front of an audience at the Joint Mathematics Meetings, and are also live streamed on the web. (You can watch the archives of past years' finals on the WWTBAM website.) Contestants will compete directly against each other in semi-final rounds, with the semi-final winners advancing to a

The National Finals are held live in front of an audience at the Joint Mathematics Meetings, and are also live streamed on the web. (You can watch the archives of past years' finals on the WWTBAM website.) Contestants will compete directly against each other in semi-final rounds, with the semi-final winners advancing to a

*Jeopardy!*-style buzz-in final round to determine a champion.
DPatrick
2017-09-27 20:32:40

The WWTBAM web site is ams.org/wwtbam and has lots more information and past years' contests.

The WWTBAM web site is ams.org/wwtbam and has lots more information and past years' contests.

mikebreen
2017-09-27 20:33:13

Everyone wins something, so we hope it's not too stressful.

Everyone wins something, so we hope it's not too stressful.

GeronimoStilton
2017-09-27 20:33:56

Even the people who get knocked out in round 1?

Even the people who get knocked out in round 1?

DPatrick
2017-09-27 20:34:04

No, I think he was talking about the 12 finalists.

No, I think he was talking about the 12 finalists.

DPatrick
2017-09-27 20:34:21

It can be stressful to be in front of a live audience and being live-streamed on the web at the same time.

It can be stressful to be in front of a live audience and being live-streamed on the web at the same time.

mikebreen
2017-09-27 20:34:23

Yes.

Yes.

DPatrick
2017-09-27 20:34:31

But everyone among the final 12 will win prizes just for making it to the National Finals.

But everyone among the final 12 will win prizes just for making it to the National Finals.

TheBoredKid
2017-09-27 20:35:45

What if there are ties for the top in each region?

What if there are ties for the top in each region?

DPatrick
2017-09-27 20:35:50

There are tiebreaking procedures.

There are tiebreaking procedures.

mikebreen
2017-09-27 20:35:57

We have a tie-breaking question in Round 2.

We have a tie-breaking question in Round 2.

DPatrick
2017-09-27 20:36:36

For example, last year's tiebreaker question was:

For example, last year's tiebreaker question was:

DPatrick
2017-09-27 20:36:41

10. What is the smallest (minimum) value of $n$ such that $(n+1)!$ has four more digits than $n!$?

10. What is the smallest (minimum) value of $n$ such that $(n+1)!$ has four more digits than $n!$?

*Note: this is the tie-breaking question. In the event of a tie for high scores in a region, the winner is the person with the closest to the correct answer without going over.*
DPatrick
2017-09-27 20:37:05

This is almost impossible to get exactly without a computer. But with a little ingenuity you can make a pretty good estimate.

This is almost impossible to get exactly without a computer. But with a little ingenuity you can make a pretty good estimate.

DaniyalQazi2
2017-09-27 20:37:30

what are the prizes for making the finals?

what are the prizes for making the finals?

DPatrick
2017-09-27 20:37:41

It's $10000 to the winner.

It's $10000 to the winner.

DPatrick
2017-09-27 20:37:57

(I think that's split halfway between the student and the school.)

(I think that's split halfway between the student and the school.)

DPatrick
2017-09-27 20:38:12

Plus other cash prizes and prizes from some of the other sponsors to the other students.

Plus other cash prizes and prizes from some of the other sponsors to the other students.

mikebreen
2017-09-27 20:38:19

Yes. Everyone gets at least $500 plus matching amount for math dept. of school.

Yes. Everyone gets at least $500 plus matching amount for math dept. of school.

Fluxed
2017-09-27 20:39:14

Why is the website so hard to navigate? Do you have web developers?

Why is the website so hard to navigate? Do you have web developers?

GeronimoStilton
2017-09-27 20:39:34

Where are the past WWTBAM problems?

Where are the past WWTBAM problems?

DPatrick
2017-09-27 20:40:13

Those are the summaries of past years.

Those are the summaries of past years.

mikebreen
2017-09-27 20:40:19

Videos linked from http://www.ams.org/wwtbam. Past tests linked to from there, too.

Videos linked from http://www.ams.org/wwtbam. Past tests linked to from there, too.

DPatrick
2017-09-27 20:40:39

...has all the past tests. I see 2018 Round One is already posted there.

...has all the past tests. I see 2018 Round One is already posted there.

DPatrick
2017-09-27 20:41:03

(Which is the contest we just discussed. The finals are in January 2018.)

(Which is the contest we just discussed. The finals are in January 2018.)

Fluxed
2017-09-27 20:41:09

Who won last year?

Who won last year?

mikebreen
2017-09-27 20:41:26

Graham O'Donnell from FL.

Graham O'Donnell from FL.

DPatrick
2017-09-27 20:42:01

DPatrick
2017-09-27 20:42:52

And here's Graham with a giant check at his school:

And here's Graham with a giant check at his school:

DPatrick
2017-09-27 20:43:00

mikebreen
2017-09-27 20:43:42

He and his family drove there under threat of ice storm. One contestant drove from Chicago (because of ice storm)!

He and his family drove there under threat of ice storm. One contestant drove from Chicago (because of ice storm)!

DPatrick
2017-09-27 20:44:15

That's right, I remember: last year in Atlanta at the finals there was a significant ice storm the night before. It was hard to get in and out of town, but everyone made it!

That's right, I remember: last year in Atlanta at the finals there was a significant ice storm the night before. It was hard to get in and out of town, but everyone made it!

DPatrick
2017-09-27 20:44:26

I can guarantee that this will not be a problem this year here in San Diego.

I can guarantee that this will not be a problem this year here in San Diego.

mikebreen
2017-09-27 20:44:58

We're holding you to that.

We're holding you to that.

DPatrick
2017-09-27 20:45:08

I think with that, we'll call it a night. Thanks for coming, and hopefully we'll see you on October 26 for our Round 2 Math Jam!

I think with that, we'll call it a night. Thanks for coming, and hopefully we'll see you on October 26 for our Round 2 Math Jam!