Prove the formula, where m and n are positive integers

a) Integral(-pi to pi) sin(mx) cos(nx)dx = 0.

b) Integral(-pi to pi) sin(mx)sin(nx)dx = 0 if m != n

Integral(-pi to pi) sin(mx)sin(nx)dx = pi if m = n

c) Integral(-pi to pi) cos(mx) cos(nx) dx = 0 if m != n

Integral(-pi to pi) cos(mx) cos(nx) dx = pi if m = n

I don't even know where to start with this problem!!! :o/