Mock AIME 2 2006-2007 Problems/Problem 10

Problem

Find the number of solutions, in degrees, to the equation $\sin^{10}x + \cos^{10}x = \frac{29}{16}\cos^4 2x,$ where $0^\circ \le x^\circ \le 2007^\circ.$

Solution

We know $\cos2x=2\cos^2x-1=1-2\sin^2x$

So, $\left(\frac{1-\cos2x}2\right)^5+\left(\frac{1+\cos2x}2\right)^5=\frac{29}{16}\cos^42x$

On Simplification, $24\cos^42x-10\cos^22x-1=0\implies 24(\cos^22x)^2-10\cos^22x-1=0$

So, $\cos^22x=\frac{10\pm \sqrt{10^2-4\cdot24\cdot(-1)}}{2\cdot24}=\frac12$ or $-\frac1{12}$

As $x$ is real, $\cos^22x\ge 0\implies \cos^22x=\frac12\implies 2\cos^22x=1$

Hence, $\cos4x=0\implies 4x=(2n+1)90^\circ$ or $x=(2n+1)22.5^\circ$ where $n$ is any integer.

So, $0\le \frac{(2n+1)90}4\le2007\implies -\frac12\le n\le \frac{882}{20}<45\implies 0\le n\le44$

Mock AIME 2 2006-2007 (Problems, Source)
Preceded by Problem 9	Followed by Problem 11
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Mock AIME 2 2006-2007 Problems/Problem 10

Problem

Solution

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