Difference between revisions of "2006 AMC 12A Problems/Problem 24"
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The expression | The expression | ||
− | < | + | <cmath>(x+y+z)^{2006}+(x-y-z)^{2006}</cmath> |
is simplified by expanding it and combining like terms. How many terms are in the simplified expression? | is simplified by expanding it and combining like terms. How many terms are in the simplified expression? | ||
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terms without cancellation. For any term in the second expansion to be negative, the parity of the exponents of <math>y</math> and <math>z</math> must be opposite. Now we find a pattern: | terms without cancellation. For any term in the second expansion to be negative, the parity of the exponents of <math>y</math> and <math>z</math> must be opposite. Now we find a pattern: | ||
− | if the exponent of <math>y</math> is 1, the exponent of <math>z</math> can be all even integers up to 2004, so there are 1003 terms. | + | if the exponent of <math>y</math> is <math>1</math>, the exponent of <math>z</math> can be all even integers up to <math>2004</math>, so there are <math>1003</math> terms. |
− | if the exponent of <math>y</math> is 3, the exponent of <math>z</math> can go up to 2002, so there are 1002 terms. | + | if the exponent of <math>y</math> is <math>3</math>, the exponent of <math>z</math> can go up to <math>2002</math>, so there are <math>1002</math> terms. |
<math>\vdots</math> | <math>\vdots</math> | ||
− | if the exponent of <math>y</math> is 2005, then <math>z</math> can only be 0, so there is 1 term. | + | if the exponent of <math>y</math> is <math>2005</math>, then <math>z</math> can only be 0, so there is <math>1</math> term. |
If we add them up, we get <math>\frac{1003\cdot1004}{2}</math> terms. However, we can switch the exponents of <math>y</math> and <math>z</math> and these terms will still have a negative sign. So there are a total of <math>1003\cdot1004</math> negative terms. | If we add them up, we get <math>\frac{1003\cdot1004}{2}</math> terms. However, we can switch the exponents of <math>y</math> and <math>z</math> and these terms will still have a negative sign. So there are a total of <math>1003\cdot1004</math> negative terms. | ||
− | By subtracting this number from 2015028, we obtain <math>\boxed{D}</math> or <math> | + | By subtracting this number from 2015028, we obtain <math>\boxed{\mathrm{D}}</math> or <math>1,008,016</math> as our answer. |
==Solution 2== | ==Solution 2== | ||
− | Alternatively, we can use a generating function to solve this problem. | + | Alternatively, we can use a [[generating function]] to solve this problem. |
The goal is to find the generating function for the number of unique terms in the simplified expression (in terms of <math>k</math>). In other words, we want to find <math>f(x)</math> where the coefficient of <math>x^k</math> equals the number of unique terms in <math>(x+y+z)^k + (x-y-z)^k</math>. | The goal is to find the generating function for the number of unique terms in the simplified expression (in terms of <math>k</math>). In other words, we want to find <math>f(x)</math> where the coefficient of <math>x^k</math> equals the number of unique terms in <math>(x+y+z)^k + (x-y-z)^k</math>. | ||
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<math>(x+P)^{2006}+(x-P)^{2006}</math>. | <math>(x+P)^{2006}+(x-P)^{2006}</math>. | ||
− | Expanding this using binomial theorem gives <math>x^n+ | + | Expanding this using binomial theorem gives <math>(x^n+Px^{n-1}+...+P^{n-1}x+P^n)+(x^n-Px^{n-1}+...-P^{n-1}x+P^n)</math>, where <math>n=2006</math> (we may omit the coefficients, as we are seeking for the number of terms, not the terms themselves). |
− | Simplifying gives: <math>2(x^n+x^{n-2} | + | Simplifying gives: <math>2(x^n+x^{n-2}P^2+...+x^2P^{n-2}+P^n)</math>. Note that two terms that come out of different powers of <math>P</math> cannot combine and simplify, as their exponent of <math>x</math> will differ. Therefore, we simply add the number of terms produced from each addend. By the Binomial Theorem, <math>P^k=(y+z)^k</math> will have <math>k+1</math> terms, so the answer is <math>1+3+5+...+2007=1004^2=1,008,016 \implies \boxed{D}</math>. |
− | + | ==Solution 4== | |
− | <math> | + | Using stars and bars we know that <math>(x+y+z)^{2006}</math> has <math>{2006+2\choose 2} |
+ | </math> or <math>2015028</math> different terms. Now we need to find out how many of these terms are canceled out by <math>(x-y-z)^{2006}</math>. We know that for any term(let's say <math>x^{a}(-y)^{b}(-z)^{c}</math>) where <math>a+b+c=2006</math> of the expansion of <math>(x-y-z)^{2006}</math> is only going to cancel out with the corresponding term <math>x^{a}y^{b}z^{c}</math> if only <math>b</math> is odd and <math>c</math> is even or <math>b</math> is even and <math>c</math> is odd. Now let's do some casework to see how many terms fit this criteria: | ||
− | + | Case 1: <math>a</math> is even | |
− | We can | + | Now we know that <math>a</math> is even and <math>a+b+c=2006</math>. Thus <math>b+c</math> is also even or both <math>b</math> and <math>c</math> are odd or both <math>b</math> and <math>c</math> are even. This case clearly fails the above criteria and there are 0 possible solutions. |
+ | |||
+ | Case 2: <math>a</math> is odd | ||
+ | |||
+ | Now we know that <math>a</math> is odd and <math>a+b+c=2006</math>. Thus <math>b+c</math> is odd and <math>b</math> is odd and <math>c</math> is even or <math>b</math> is even and <math>c</math> is odd. All terms that have <math>a</math> being odd work. | ||
+ | |||
+ | We now need to figure out how many of the terms have <math>a</math> as a odd number. We know that <math>a</math> can be equal to any number between 0 and 2006. There are 1003 odd numbers between this range and 2007 total numbers. Thus <math>\frac{1003}{2007}</math> of the <math>2015028</math> terms will cancel out and <math>\frac{1004}{2007}</math> of the terms will work. Thus there are <math>(\frac{1004}{2007})(2015028)</math> terms. This number comes out to be <math>1,008,016</math> <math>\implies \boxed{D}</math> | ||
+ | (Author: David Camacho) | ||
+ | |||
+ | ==Solution 5== | ||
+ | Noticing how <math>y</math> and <math>z</math> are negative in the second part of the expression, let <math>x=a</math> and <math>y+z = b</math>. Then we get <cmath>(a+b)^{2006} + (a-b)^{2006}</cmath> | ||
+ | We know that the terms that don't cancel out have even powers of <math>a</math> and <math>b</math>. Our expansion will be in the form of | ||
+ | <cmath>2a^{2006} + x_1a^{2004}b^{2} + x_2a^{2002}b^{4} + \cdots + 2b^{2006}</cmath> | ||
+ | Note that <math>b^n = (x+y)^n</math> has <math>n+1</math> terms. Furthermore, the current expression is irreducible as each term has a different <math>x</math> power. Thus, when we write <math>a</math> and <math>b</math> back to their original terms, we will have <math>1+3+5+ \cdots + 2007 = 1004^2 = 1,008,016 \implies \boxed{D}</math> | ||
+ | |||
+ | -smartguy888 | ||
== See also == | == See also == | ||
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{{AMC12 box|year=2006|ab=A|num-b=23|num-a=25}} | {{AMC12 box|year=2006|ab=A|num-b=23|num-a=25}} | ||
+ | {{MAA Notice}} | ||
[[Category:Introductory Algebra Problems]] | [[Category:Introductory Algebra Problems]] | ||
[[Category:Introductory Combinatorics Problems]] | [[Category:Introductory Combinatorics Problems]] | ||
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Latest revision as of 17:37, 17 September 2023
Problem
The expression
is simplified by expanding it and combining like terms. How many terms are in the simplified expression?
Solution 1
By the Multinomial Theorem, the summands can be written as
and
respectively. Since the coefficients of like terms are the same in each expression, each like term either cancel one another out or the coefficient doubles. In each expansion there are:
terms without cancellation. For any term in the second expansion to be negative, the parity of the exponents of and
must be opposite. Now we find a pattern:
if the exponent of is
, the exponent of
can be all even integers up to
, so there are
terms.
if the exponent of is
, the exponent of
can go up to
, so there are
terms.
if the exponent of is
, then
can only be 0, so there is
term.
If we add them up, we get terms. However, we can switch the exponents of
and
and these terms will still have a negative sign. So there are a total of
negative terms.
By subtracting this number from 2015028, we obtain or
as our answer.
Solution 2
Alternatively, we can use a generating function to solve this problem.
The goal is to find the generating function for the number of unique terms in the simplified expression (in terms of ). In other words, we want to find
where the coefficient of
equals the number of unique terms in
.
First, we note that all unique terms in the expression have the form, , where
and
is some constant. Therefore, the generating function for the MAXIMUM number of unique terms possible in the simplified expression of
is
Secondly, we note that a certain number of terms of the form, , do not appear in the simplified version of our expression because those terms cancel. Specifically, we observe that terms cancel when
because every unique term is of the form:
for all possible
.
Since the generating function for the maximum number of unique terms is already known, it is logical that we want to find the generating function for the number of terms that cancel, also in terms of . With some thought, we see that this desired generating function is the following:
Now, we want to subtract the latter from the former in order to get the generating function for the number of unique terms in , our initial goal:
which equals
The coefficient of of the above expression equals
Evaluating the expression, we get , as expected.
Solution 3
Define such that
. Then the expression in the problem becomes:
.
Expanding this using binomial theorem gives , where
(we may omit the coefficients, as we are seeking for the number of terms, not the terms themselves).
Simplifying gives: . Note that two terms that come out of different powers of
cannot combine and simplify, as their exponent of
will differ. Therefore, we simply add the number of terms produced from each addend. By the Binomial Theorem,
will have
terms, so the answer is
.
Solution 4
Using stars and bars we know that has
or
different terms. Now we need to find out how many of these terms are canceled out by
. We know that for any term(let's say
) where
of the expansion of
is only going to cancel out with the corresponding term
if only
is odd and
is even or
is even and
is odd. Now let's do some casework to see how many terms fit this criteria:
Case 1: is even
Now we know that is even and
. Thus
is also even or both
and
are odd or both
and
are even. This case clearly fails the above criteria and there are 0 possible solutions.
Case 2: is odd
Now we know that is odd and
. Thus
is odd and
is odd and
is even or
is even and
is odd. All terms that have
being odd work.
We now need to figure out how many of the terms have as a odd number. We know that
can be equal to any number between 0 and 2006. There are 1003 odd numbers between this range and 2007 total numbers. Thus
of the
terms will cancel out and
of the terms will work. Thus there are
terms. This number comes out to be
(Author: David Camacho)
Solution 5
Noticing how and
are negative in the second part of the expression, let
and
. Then we get
We know that the terms that don't cancel out have even powers of
and
. Our expansion will be in the form of
Note that
has
terms. Furthermore, the current expression is irreducible as each term has a different
power. Thus, when we write
and
back to their original terms, we will have
-smartguy888
See also
2006 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 23 |
Followed by Problem 25 |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.