We know that-

Now,

L.H.S = sin 3x + sin 2x - sin x

= (sin 3x - sin x) + sin 2x

= 2 cos 2x sin x + 2 sin x cos x [∵ sin 2x = 2 sin x cos x]

= 2 sin x (cos 2x + cos x)

= R.H.S

Hence, L.H.S = R.H.S…Proved