I am trying to work through some examples we have been given on flux out of a cube but am having difficulty in seeing how one one line of the answer becomes the next. The question is analysing the flux out of a cube by looking at each side individually and working out the surface integrals. Looking at side a a double integral is formed, which I can get to and understand, but I dont not understand how this has been evaluated. The formula is in the attached .bmp I get the answer of 1/3 when I try to evaluate this. I do not see why in the second line the Int dz has been seperated out, or where the 1/2 has appeared from. Many thanks for any help. PS. apologies for not writing it in LaTeX but I don't know how yet and was hoping for a quick answer.