Take L.H.S

cos 20° cos 40° cos 80°

Multiplying & Dividing by 2:

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

Multiplying & Dividing by 2:

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

{∵ cos (180° - A) = - cos A}

= R.H.S

Hence Proved