Electric Field Above Square: Find E-Field at z Height

[nicksauce]

Homework Statement

Find the electric field a height z above a square of side length a with uniform surface charge \sigma

Homework Equations

The Attempt at a Solution

E=\frac{z\sigma}{4\pi\epsilon_0}\int^{a/2}_{-a/2}\int^{a/2}_{-a/2}\frac{dx dy}{(x^2+y^2+z^2)^{3/2}}
(in the z direction). However, unless I maple'd wrong, this gives the wrong answer. Can you anyone see a problem with the way the integral is set up?

 

[learningphysics]

Looks right to me.