Prove that:
Take L.H.S
cos 40° cos 80° cos 160°
Multiplying & Dividing by 2:
{∵ 2 cos A cos B = cos (A + B) + cos (A – B)}
{∵ cos (180° - A) = - cos A & cos (180° + A) = - cos A}
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