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Question

Prove that:
(sin3x+sinx)sinx+(cos3xcosx)cosx=0

Solution
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LHS=(sin3x+sinx)sinx+(cos3xcosx)cosx

=[2sin(3x+x2)cos(3xx2)]sinx+[2sin(3x+x2)sin(3xx2)]cosx

2[sinxcosxsin2xsinxcosxsin2x]=0=RHS

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