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Prove that the equation is an identity.

cos x - cos 5x= 4 sin 3x sin x cos x

1 answer

4sin3x sin x cos x
using double-angle formula,
2sin3x sin2x
using product-to-sum formula,
2(1/2 cos(3x-2x)-cos(3x+2x))
cos x - cos 5x
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