Prove that :
Take L.H.S.
sin 3A + sin 2A – sin A
= (sin 3A – sin A) + sin 2A
{sin 2A = 2 sin A cos A}
= 2 cos 2A sin A + 2 sin A cos A
= 2 sin A (cos 2A + cos A)
= R.H.S.
Hence Proved