Rewrite cos 3 Theta - cos theta as a product

Question

Could you show me how to use the sum to product method to answer this? Thanks!

Answer 1

cos u + cos v = 2 cos(½(u+v)) cos(½(u−v))

cos u − cos v = −2 sin(½(u+v)) sin(½(u−v))

sin u + sin v = 2 sin(½(u+v)) cos(½(u−v))

sin u − sin v = 2 sin(½(u−v)) cos(½(u+v))

 

You will need to use the second of these

 

cos3θ-cosθ=-2sin2θsinθ