Question

Cos(3x) = cos^2x - 3sin^2x cosx

Answer 1

`\cos3x=\cos(2x+x)=\cos2x\cos x-\sin2x\sin x`

`=(\cos^2x-\sin^2x)\cos x-2\sin^2x\cos x`

`=\cos^3x-3\sin^2x\cos x`