M 4.1

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

$\sin x+\sin y=2\sin \frac{x+y}{2}\cos \frac{x-y}{2}$

$\sin x-\sin y=2\cos \frac{x+y}{2}\sin \frac{x-y}{2}$

$\frac{\sin x}{\cos x}=\tan x$