Double-Angle Identities

$\displaystyle \sin 2x=2\sin x \cos x$

$\displaystyle \cos 2x=\left\{\begin{aligned}& \cos^2x-\sin^2x & & 1-2\sin^2x & & 2\cos^2x-1 & \end{aligned}\right.$

$\displaystyle \tan 2x=\left\{\begin{aligned}& \frac{2\tan x}{1-\tan^2x} & [7... ...\cot^2x-1} & [7pt] & \frac{2}{\cot x-\tan x} & [7pt] \end{aligned}\right.$