Answer :

Take L.H.S:


cos 20° cos 100° + cos 100° cos 140° - cos 140° cos 200°


Multiplying & Dividing by 2:



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





{cos (180° + A) = - cos A ;


cos (90° + A) = - sin A &


cos (360° - A) = cos A}












= R.H.S.


Hence Proved


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