Q3 of 90 Page 8

Prove that :

cos 55° + cos 65° + cos 175° = 0

Take L.H.S:


cos 55° + cos 65° + cos 175°





{cos (180° - A) = - cos A}


= 2 cos 60° cos (-5°) – cos 5°


{cos (-A) = cos A}



= cos 5° – cos 5°


= 0


= R.H.S


Hence Proved


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