cos(3x) = cos^2x - 3sin^2x cosx
\(\cos3x=\cos(2x+x)=\cos2x\cos x-\sin2x\sin x\)\(=(\cos^2x-\sin^2x)\cos x-2\sin^2x\cos x\)\(=\cos^3x-3\sin^2x\cos x\)
