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

Question

Anybody have  an idea how to solve that question... think needs to simplify first and use identities to solve.. but cant even start it..

 

please any help is welcome..

Answer 1

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.