prove the following integrals:

a)sin3xcos7xdx = -1/20cos(10x)+1/8cos(4x)

b)sin8xcos3xdx = -1/22cos(11x)-1/10cos(5x)

In the first, cos7x= cos(4x+3x) then you have an identity to reduce that .

In the second, change sin8x to sin(5x+3x) to its identity.