1. The problem statement, all variables and given/known data http://www.brookscole.com/math_d/sp...athematica_labs/14-multipleintegrals/p05a.pdf 2. Relevant equations 3. The attempt at a solution My question concerns the (1) and (2) next to the figure of the rotating coordinate system. x=ucos(theta)-vsin(theta) y=ucos(theta)..... ..... So my problem is I can't visualize it. I can visualize that u = ucos(theta) + usin(theta), but I don't get how u = xcos(theta) + ysin(theta). If anyone can explain or try to visually show it to me would be great. Thanks.