Q3 of 90 Page 8

Prove that :

sin 80° – cos 70° = cos 50°

sin 80° – cos 70° = cos 50°


⇒ sin 80° = cos 50° + cos 70°


Take RHS:


cos 50° + cos 70°





= 2 cos 60° cos (-10°)


{cos (-A) = cos A}



= cos 10°


= cos (90° - 80°)


{cos (90° - A) = sin A}


= sin 80°


= L.H.S


Hence Proved


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