for integration in two variables, say x and y let the original formula have dxdy when can i change it into dydx? and how to do that ? And also: Why ? the only reason for doing that i know till now is for example i have a function that is easy-to-integrate w.r.t. y but not w.r.t x, so i do make dy before dx to integrate the function w.r.t y then x, instead if x then y But how i do that ? and When ? And Why ( Another reasons .. ) ?