cos(3x-2y)cos(x-2y)+sin(3x-2y)sin(x-2y)

Question

cos(3x-2y)cos(x-2y)+sin(3x-2y)sin(x-2y)

Answer 1

 cos(3x-2y)cos(x-2y)+sin(3x-2y)sin(x-2y)= cos((3х-2у)-(х-2у))= cos(3х-2у-х+2у)=cos2х