When you have a DiffEQ in the form Mdx + Ndy = 0 and you want an exact solution, sometimes you need to multiply by an integration factor p(x,y). The book tells me that the integration factor will either be p =(x^m)(y^n), p = f(x) or p = f(y) and then it gives me the procedure used to solve for all of these cases. Very straightforward. But how besides insight or guess and check can I decide which type of integration factor to use? Are there any tricks, rules or patterns? Let me know if I should clarify further.