Difference between revisions of "2008 AIME II Problems/Problem 8"
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===Solution 1=== | ===Solution 1=== | ||
− | By the [[trigonometric identity|product-to-sum identities]], we have that <math>2\cos a \sin b = \sin (a+b) - \sin (a-b)</math>. Therefore, this reduces to a | + | By the [[trigonometric identity|product-to-sum identities]], we have that <math>2\cos a \sin b = \sin (a+b) - \sin (a-b)</math>. Therefore, this reduces to a telescope series: |
<cmath>\begin{align*} | <cmath>\begin{align*} | ||
\sum_{k=1}^{n} 2\cos(k^2a)\sin(ka) &= \sum_{k=1}^{n} [\sin(k(k+1)a) - \sin((k-1)ka)]\\ | \sum_{k=1}^{n} 2\cos(k^2a)\sin(ka) &= \sum_{k=1}^{n} [\sin(k(k+1)a) - \sin((k-1)ka)]\\ | ||
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\end{align*}</cmath> | \end{align*}</cmath> | ||
− | Thus, we need <math>\sin \left(\frac{n(n+1)\pi}{2008}\right)</math> to be an integer; this can be only <math>\{-1,0,1\}</math>, which occur when <math>2 \cdot \frac{n(n+1)}{2008}</math> is an integer. Thus <math>1004 = 2^2 \cdot 251 | n(n+1) \Longrightarrow 251 | n, n+1</math>. | + | Thus, we need <math>\sin \left(\frac{n(n+1)\pi}{2008}\right)</math> to be an integer; this can be only <math>\{-1,0,1\}</math>, which occur when <math>2 \cdot \frac{n(n+1)}{2008}</math> is an integer. Thus <math>1004 = 2^2 \cdot 251 | n(n+1) \Longrightarrow 251 | n, n+1</math>. We know that <math>n</math> cannot be <math>250</math> as <math>250</math> isn't divisible by <math>4</math>, so 1004 doesn't divide <math>n(n+1) = 250 \cdot 251</math>. Therefore, it is clear that <math>n = \boxed{251}</math> is the smallest such integer. |
===Solution 2=== | ===Solution 2=== | ||
− | We proceed with complex trigonometry. We know that for all <math>\theta</math>, we have <math>\cos \theta = \dfrac{1}{2} \left( z + \dfrac{1}{z} \right)</math> and <math>\sin \theta = \dfrac{1}{2i} \left( z - \dfrac{1}{z} \right)</math> for some complex number <math>z</math>. Similarly, we have <math>\cos n \theta = \dfrac{1}{2} \left( z^n + \dfrac{1}{z^n} \right)</math> and <math>\sin n \theta = \dfrac{1}{2i} \left(z^n - \dfrac{1}{z^n} \right)</math>. Thus, we have <math>\cos n^2 a \sin n a = \dfrac{1}{4i} \left( z^{n^2} + \dfrac{1}{z^{n^2}} \right) \left( z^{n} - \dfrac{1}{z^n} \right)</math> | + | We proceed with complex trigonometry. We know that for all <math>\theta</math>, we have <math>\cos \theta = \dfrac{1}{2} \left( z + \dfrac{1}{z} \right)</math> and <math>\sin \theta = \dfrac{1}{2i} \left( z - \dfrac{1}{z} \right)</math> for some complex number <math>z</math> on the unit circle. Similarly, we have <math>\cos n \theta = \dfrac{1}{2} \left( z^n + \dfrac{1}{z^n} \right)</math> and <math>\sin n \theta = \dfrac{1}{2i} \left(z^n - \dfrac{1}{z^n} \right)</math>. Thus, we have <math>\cos n^2 a \sin n a = \dfrac{1}{4i} \left( z^{n^2} + \dfrac{1}{z^{n^2}} \right) \left( z^{n} - \dfrac{1}{z^n} \right)</math> |
<math>= \dfrac{1}{4i} \left( z^{n^2 + n} - \dfrac{1}{z^{n^2 + n}} - z^{n^2 - n} + \dfrac{1}{z^{n^2 - n}} \right)</math> | <math>= \dfrac{1}{4i} \left( z^{n^2 + n} - \dfrac{1}{z^{n^2 + n}} - z^{n^2 - n} + \dfrac{1}{z^{n^2 - n}} \right)</math> |
Latest revision as of 17:26, 16 January 2023
Problem
Let . Find the smallest positive integer such that is an integer.
Solution
Solution 1
By the product-to-sum identities, we have that . Therefore, this reduces to a telescope series:
Thus, we need to be an integer; this can be only , which occur when is an integer. Thus . We know that cannot be as isn't divisible by , so 1004 doesn't divide . Therefore, it is clear that is the smallest such integer.
Solution 2
We proceed with complex trigonometry. We know that for all , we have and for some complex number on the unit circle. Similarly, we have and . Thus, we have
which clearly telescopes! Since the outside the brackets cancels with the inside, we see that the sum up to terms is
.
This expression takes on an integer value iff is an integer; that is, . Clearly, , implying that . Since we want the smallest possible value of , we see that we must have . If , then we have , which is clearly not divisible by . However, if , then , so our answer is .
It should be noted that the product-to-sum rules follow directly from complex trigonometry, so this solution is essentially equivalent to the solution above.
See also
2008 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 7 |
Followed by Problem 9 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.