I have to find the electrical field in the center (of the base) of a semi spherical shell of radius R. The total charge Q (Q > 0) is uniform on the intern surface of the semi sphere. Here's a scheme:

![enter image description here](https://i.stack.imgur.com/jIl1j.png)

And here's the solution:

![enter image description here](https://i.stack.imgur.com/al7wf.png)

I found the first equality ($E\_{z}$ = the first integral), but then I was stuck to integrate other the intern surface of the semi-sphere. 

Could someone explain me how it's done? I don't get the second equality.

Many thanks,