Remember the chain rule?Thank you for your reply. It is an actual example. I have to solve a system of differential equations using Newton-Raphson method, so, let's say I have a function: f1(y1(x1),y2(x1,x2)) = 0, with something like f1 =dy1/dx1 + something else. Then to construct the Jacobian I'll need to evaluate ∂f1/∂y1, which will lead to ∂(dy1/dx1)/∂y1. Any leads?