cos x = 0.6. Use the triple-angle formula to find cos 3x.

$\sin 2x = 2\sin x \cos x$; $\cos 2x = \cos^2 x - \sin^2 x = 2\cos^2 x - 1 = 1 - 2\sin^2 x$; $\tan 2x = \dfrac{2\tan x}{1 - \tan^2 x}$. Triple angles: $\sin 3x = 3\sin x - 4\sin^3 x$ and $\cos 3x = 4\cos^3 x - 3\cos x$.