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ModusPwnd
#2
Jan31-13, 04:19 PM
P: 1,105
Both sides were squared in that step. So on the right you have the integral times the integral. Remember that the variable you integrate over is a "dummy variable", that means it can be changed to a different character and it doesnt matter. http://mathworld.wolfram.com/DummyVariable.html

By changing the dummy variable from "x" to "y" it is better illustrated how you are going from cartesian to polar coordinates.