4. Exact ODE Total Differential $ u(x,y)=c $ $ du=\frac{\partial u}{\partial x}dx+\frac{\partial u}{\partial y}dy=u_{x}dx+u_{y}dy=0 $ Definition $ M(x,y)dx+N(x,y)dy=0 $ is called an Exact ODE $ \exists u(x,y) $ s.t. $ u_{x}=M, u_{y}=N $ Theorem $ M(x,y)dx+N(x,y)dy=0 $ is exact $ \Leftrightarrow M_{y}=N_{x} $ $ u=\int M dx+k(y)=\int N dy+l(x) $ ex) $ (e^{x}+y)dx+(x-e^{-y})dy=0 $ $ M=e^{x}+y, N=x-..