Which expression is equivalent to cos(4x)?

Use the double-angle identity cos(2θ) = 2cos²(θ) − 1. Applying it first gives cos(2x) = 2cos²(x) − 1; applying it again gives cos(4x) = 2cos²(2x) − 1 = 2(2cos²(x) − 1)² − 1. This is choice A.

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