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.
prove the following integrals:
a)sin3xcos7xdx = -1/20cos(10x)+1/8cos(4x)
b)sin8xcos3xdx = -1/22cos(11x)-1/10cos(5x)
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