Question

Any one know how to prove the "Potential function theorem" used in solving exact differential equations?
Here's the theorem;
M(x,y)+N(x,y)dy/dx=0 , is an exact ODE then, Ǝ a function P(x,y) s.t. ∂P/∂x=M(x,y) &
∂P/∂y=N(x,y)