I must be doing something wrong here.

Um I think that wouldn't work.

Let's say that y = x^5 or something, and that u = x^5, therefore y = u.

y' = 1 ????/??/

(Actually it would be more like dy/du = 1. Don't ask me what that means.)


As for CoB's question, I have never encountered that, so I can't help there.

We don't want dy/dx. we want dy/du. When x^5 increases by a, how much does y increase? By a.