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Question

Prove that: 7π12+cosπ12=sin5π12-sinπ12

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Solution

105°=7π12, 15°=π12, 75°=5π12, 15°=π12
LHS = cos105o + cos15o
= cos(90o + 15o) + cos(90o - 75o)
= - sin 15o + sin 75o [As cos(90o+A) = - sin A and cos(90o - B) = sin B]
= sin 75o - sin 15o
= RHS
Hence proved.

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