Q3 of 90 Page 8

Prove that :

cos 80° + cos 40° – cos 20° = 0

Take L.H.S:


cos 80° + cos 40° – cos 20°





= 2 cos 60° cos 20° – cos 20°



= cos 20° – cos 20°


= 0


= R.H.S


Hence Proved


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