# Partial derivatives of composition

1. Mar 11, 2010

### cantgetright

1. The problem statement, all variables and given/known data

Find the partial derivatives with respect to $u,v$ of $\bar{U}(\bar{x}(u,v))$, where $\bar{U}$ is the unit normal to a surface given by the parametrization $\bar{x}(u,v)$. (This, of course, is part of a larger problem, but I just am looking for advice with the calculus.)

2. Relevant equations

Chain rule?

3. The attempt at a solution

I am showing some properties of the shape operator, so everything is in "general terms" (that's why there's no explicit expression for the unit normal or the parametrization). I'm used to explicit expressions--e.g., where I could evaluate what the composition looked like explicitly and take partial derivatives. Here, I am at a loss. I would like to take derivative of U evaluated at x(u,v) with respect to u and then multiply by derivative of x(u,v) with respect to u (and then repeat the process for the partial derivative with respect to v), but in "general terms", how do I express the partial derivative of U?

I am sorry if this is so elementary, but I have not seen calculus in almost 20 years. Be gentle.