$sin3x=3sinx-4sin^3x\\\\L=sin(2x+x)=sin2xcosx+sinxcos2x\\\\=2sinxcosxcosx+sinx(cos^2x-sin^2x)\\\\=2sinxcos^2x+sinxcos^2x-sin^3x\\\\=3sinxcos^2x-sin^3x\\\\=3sinx(1-sin^2x)-sin^3x\\\\=3sinx-3sin^3x-sin^3x\\\\=3sinx-4sin^3x=R$

Umer Kiani answered Oct 18, 2015

Derive identity sin(3x) = 3 sinx−4 sin³x

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