1. The problem statement, all variables and given/known data Hi. I think it will be easiest to understand if I upload a word document as it has some complicated characters. The bold letters stand for vectors. http://www.sendspace.pl/file/1f830d4ff025d966f71b62c [Broken] 2. Relevant equations Divergence theorm http://www.math.oregonstate.edu/hom...ulusQuestStudyGuides/vcalc/diverg/diverg.html 3. The attempt at a solution I tried to change the form of the those two equations to the form stated in divergence theorm and then to compare the u (or F as in link above), but the u (F) on the left hand side is never equal to this on the right.