Prove that

Submitted by alpha on Thu, 07/02/2009 - 17:06.

$\sin3A+\sin2A-\sin A=4\sin A \cos\frac{A}{2}\cos\frac{3A}{2}$

$\sin3A+\sin2A-\sin A=\sin3A-\sin A+\sin2A=2 \sin A\cos 2A+2\sin A \cos A=2\sin A(\cos 2A+\cos A)=4\sin A \cos\frac{A}{2}\cos\frac{3A}{2}$