Answer :
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
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