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2006 AIME I Problems/Problem 12

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Problem

Find the sum of the values of x such that \cos^3 3x+ \cos^3 5x = 8 \cos^3 4x \cos^3 x, where x is measured in degrees and 100< x< 200.

Solution

Observe that 2\cos 4x\cos x = \cos 5x + \cos 3x by the sum-to-product formulas. Defining a = \cos 3x and b = \cos 5x, we have a^3 + b^3 = (a+b)^3 \Leftrightarrow ab(a+b) = 0. But a+b = 2\cos 4x\cos x, so we require \cos x = 0, \cos 3x = 0, \cos 4x = 0, or \cos 5x = 0.

Hence the solution set is A = \{150, 126, 162, 198, 112.5, 157.5\} and thus \sum_{x \in A} x = 906.

See also

2006 AIME I (ProblemsResources)
Preceded by
Problem 11
Followed by
Problem 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Want to learn how to tackle those tough MATHCOUNTS and AMC counting and probability problems? Check out Art of Problem Solving's Introduction to Counting & Probability by David Patrick.
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