Q3 of 90 Page 8

Prove that :

cos 20° + cos 100° + cos 140° = 0

Take L.H.S:


cos 20° + cos 100° + cos 140°





{cos (180° - A) = - cos A}


= 2 cos 60° cos (-40°) – cos 40°


{cos (-A) = cos A}



= cos 40° – cos 40°


= 0


= R.H.S


Hence Proved


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