2014 AMC 12B Problem 25

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Concepts:trigonometric identitysum of factors

Difficulty rating: 2890

25.

What is the sum of all positive real solutions xx to the equation 2cos(2x)(cos(2x)cos(2014π2x))=cos(4x)1? 2\cos(2x)\left(\cos(2x) - \cos\left(\dfrac{2014\pi^2}{x}\right)\right) = \cos(4x) - 1?

π\pi

810π810\pi

1008π1008\pi

1080π1080\pi

1800π1800\pi

Solution:

Let x=πy2.x = \tfrac{\pi y}{2}. Dividing by 22 and using 12(1cos(2πy))=sin2(πy),\tfrac12(1 - \cos(2\pi y)) = \sin^2(\pi y), the equation simplifies to cos(πy)cos(4028πy)=1. \cos(\pi y)\cos\left(\dfrac{4028\pi}{y}\right) = 1.

Both cosines must equal 11 or both equal 1,-1, so yy and 4028y\tfrac{4028}{y} are integers of the same parity. Since 4028=2219534028 = 2^2 \cdot 19 \cdot 53 is even, both must be even, so y=2ay = 2a with aa a positive odd divisor of 2014=21953,2014 = 2 \cdot 19 \cdot 53, giving a{1,19,53,1953}.a \in \{1, 19, 53, 19 \cdot 53\}.

Each such aa gives x=πy2=πa,x = \tfrac{\pi y}{2} = \pi a, so the sum of solutions is π(1+19+53+1953)=π(19+1)(53+1)=1080π. \pi(1 + 19 + 53 + 19\cdot53) = \pi(19+1)(53+1) = 1080\pi.

Thus, the correct answer is D.

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