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show that cos(A-B) - cos(A+B) = 2sinA sinB

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1 Answer

cos (A - B) = cos A cos B + sin A sin B

cos (A + B) = cos A cos B - sin A sin B

so 

cos (A - B) - cos (A + B) = [cos A cos B + sin A sin B] - [cos A cos B - sin A sin B]

Simplify and solve.