Divergence Theorem Explained: Learn the Basics

[Harmony]

http://img60.imageshack.us/img60/9696/21249035.jpg

I am stuck at the last step. Can anyone give some hints?

Thanks in advanced.

 

[gabbagabbahey]

Just choose a coordinate system so that one of your unit vectors is parallel to the surface normal, say $\hat{e}_1=\hat{n}$, and the others are orthogonal to it.