1. The problem statement, all variables and given/known data I'm trying to understand how a certain substitution can be made with regards to taking the partial derivative of a function product when the variable I am differentiating by is a function itself. 2. Relevant equations (∂/∂p) (v(p)p(x,t)) = v(p) + (∂v/∂p)p 3. The attempt at a solution My first thought was to use the product rule, but that seems to fall down with the function p(x,t) there. Am I missing something obvious here?