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Question

Given cos(AB)=cosAcosB+sinAsinB. Taking suitable A and B, find cos15.

Solution
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We have,

cos(AB)=cosAcosB+sinAsinB …… (1)

Then, cos15o=?

Let A=45o and B=30o

Put the value of A and B in equation (1) and we get,

cos(45o30o)=cos45ocos30o+sin45osin30o

cos15o=12×32+12×12

cos15o=3+122

Hence, this is the answer.

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