Show that
If you let , you'll get a integral that maybe you've seen before.
I'll offer a starting point.
Let p and q be polynomials. Then
Let and
Then
Now integrate both sides over the unit square centered at
If you switch the order of integration of the second integral on the left, which is permissible since the integrand is never negative, you'll notice that the the two integrals on the left are identical.