Fubini's Theorem
Fubini’s theorem for Double Integrals Over Rectangles is that for a rectangle $D = [a,b] \times [c,d]$ the integral can be written as:
$$ \iint_D f(x,y)dA = \int_a^b\int_c^df(x,y)dydx = \int_c^d\int_a^bf(x,y)dxdy $$
Note that the order of the integrals as well as the order of the $dx$ and $dy$ are important in this case