Question

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

Answer 1

sin(7x) = sin(x+6x)
= sinx cos6x + cosx sin6x
= sinx*cos(2*3x) + cosx*sin(2*3x)
...