int cos6xcos4xdx =

Answer 1

Use cos(A)cos(B)=(1/2)(cos(A+B)+cos(A-B))

∫ cos(6x) cos(4x) dx
= (1/2)∫ cos(10x)+cos(2x) dx
= (1/2)((1/10)sin(10x)+(1/2)sin(2x)) + C
= (1/20)sin(10x)+(1/4)sin(2z) + C