This is coming up a lot in some of my thermo HW, so I'm a little confused about whether or not I'm thinking correctly. Suppose I have a function [itex]f = f(x,a)[/itex] where a is some constant. If I take the derivative of f wrt to a, what do I get? The derivative tells you the change in the function due to some small change in a quantity, so if I want to know how f changes with respect a, how does one interpet this? a can't change, so it seems that f can't change wrt to a, so [itex] df/da = 0[/itex] seems like the only thing that makes sense, but does it actually? For example, if [itex]f(x,a) = ax + a[/itex], then should [itex]df/da = 0[/itex] or [itex]df/da = x + 1[/itex] ?