Q2 of 54 Page 312

Without using trigonometric tables, prove that:
cos275° + cos215° =1

Consider cos2 75° + cos2 15° = cos2 75° + cos2 (90 - 75)°

= cos2 75° + sin2 75°


= 1


(∵cos2θ + sin2 θ = 1)


Hence, proved.


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