Cosine Product Equation Solutions
Find all positive integer values of $ n $ that satisfy the equation
\[
\cos \Bigl( \frac{\pi}{n} \Bigr) \cos \Bigl( \frac{2\pi}{n} \Bigr)
\cos \Bigl( \frac{4\pi}{n} \Bigr) \cos \Bigl( \frac{8\pi}{n} \Bigr)
\cos \Bigl( \frac{16\pi}{n} \Bigr)
= \frac{1}{32}.\]
Enter all the solutions, separated by commas.
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
First, we multiply both sides by $ \sin \frac{\pi}{n} $:
\[\sin \frac{\pi}{n} \cos \frac{\pi}{n} \cos \frac{2 \pi}{n} \cos \frac{4 \pi}{n} \cos \frac{8 \pi}{n} \cos \frac{16 \pi}{n} = \frac{1}{32} \sin \frac{\pi}{n}.\]By the double-angle formula, $ \sin \frac{\pi}{n} \cos \frac{\pi}{n} = \frac{1}{2} \sin \frac{2 \pi}{n}, $ so
\[\frac{1}{2} \sin \frac{2 \pi}{n} \cos \frac{2 \pi}{n} \cos \frac{4 \pi}{n} \cos \frac{8 \pi}{n} \cos \frac{16 \pi}{n} = \frac{1}{32} \sin \frac{\pi}{n}.\]We can apply the double-angle formula again, to get
\[\frac{1}{4} \sin \frac{4 \pi}{n} \cos \frac{4 \pi}{n} \cos \frac{8 \pi}{n} \cos \frac{16 \pi}{n} = \frac{1}{32} \sin \frac{\pi}{n}.\]Going down the line, we eventually arrive at
\[\frac{1}{32} \sin \frac{32 \pi}{n} = \frac{1}{32} \sin \frac{\pi}{n},\]so $ \sin \frac{32 \pi}{n} = \sin \frac{\pi}{n} $.
The sine of two angles are equal if and only if either they add up to an odd multiple of $ \pi, $ or they differ by a multiple of $ 2 \pi $. Thus, either
\[\frac{33 \pi}{n} = \pi (2k + 1)\]for some integer $ k, $ or
\[\frac{31 \pi}{n} = 2 \pi k\]for some integers $ k $.
The first condition becomes $ n(2k + 1) = 33, $ so $ n $ must be a divisor of 33. These are 1, 3, 11, and 33.
The second condition becomes $ nk = \frac{31}{2}, $ which has no integer solutions.
The only step we must account for is when we multiplied both sides by $ \sin \frac{\pi}{n} $. This is zero for $ n = 1, $ and we see that $ n = 1 $ does not satisfy the original equation. Thus, the only solutions are $ \boxed{3, 11, 33} $.