Take L.H.S

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

= 2 cos x {cos 120° + cos 2x}

= 2 cos x {cos (180° - 60°) + cos 2x}

{cos (180° - A) = - cos A}

= 2 cos x (cos 2x - cos 60°)

= 2 cos 2x cos x – 2 cos x cos 60°

= cos 3x + cos x – cos x

= cos 3x = R.H.S.

Hence Proved