Prove that:

Take L.H.S

sin 20° sin 40° sin 80°

Multiplying & Dividing by 2:

{∵ 2 sin A sin B = cos (A – B) – cos (A + B)}

{∵ cos (-A) = cos A}

Multiplying & Dividing by 2:

{∵ 2 cos A sin B = sin (A + B) – sin (A – B)}

{∵ sin (-A) = - sin A}

{∵ sin (180° - A) = sin A}

= R.H.S

Hence Proved