Question

Cos(a+b) +cos(3b-a)=2cos2bcos(a-b)

Answer 1

cos(A+B)cos(A−B)≡(cosAcosB−sinAsinB)(cosAcosB+sinAsinB)≡cos2Acos2B−sin2Asin2B≡cos2A(1−sin2B)−(1−cos2A)sin2B≡cos2A−cos2Asin2B−sin2B+cos2Asin2B≡cos2A−sin2B

Answer 2

Ur answer is in the picture.

Hope it helps.