Asked by Anonymous

sin(7x)= sin(x)[cos^2(3x)-sin^2(3x)]+2cos(x)cos(3x)sin(3x)

I tried for an hour but still don't know how. Plzz help
Answered by Steve
sin(7x) = sin(x+6x)
= sinx cos6x + cosx sin6x
= sinx*cos(2*3x) + cosx*sin(2*3x)
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