Sometimes, if you take the derivative with respect to the variable in the expression (and there is only one variable), you can ‘not’ use it since, for example, dx/dx = 1. But in general, you use the chain rule for derivatives.
Reviewing for the final, we started with the earlier version of a derivative which involved limits.
The letter ‘a’ is used sometimes to denote a constant.
With multiple variables, the chain rules becomes even more important.
The general approach for a problem involving changing dimensions of a rectangle was to find an expression for a particular characteristic and then take the derivative. Certain rates were known in the problem.