dxdy = r*dr*d(theta) https://math.stackexchange.com/questions/1656814/how-to-prove-dxdy-r-dr-d-theta How to prove $dxdy = r dr d \theta$? $x = r \cos \theta$, $y = r \sin \theta$ I got $dx = \cos \theta dr - r \sin \theta d \theta $ $ dy = \sin \theta dr + r \cos \theta d \theta$ How to get $dx dy = r dr d \theta$?? I saw the same math.stackexchange.com
