I see. Then I don't understand what point you were trying to make. There are lots of different ways of testing understanding of a concept. I presented one way of doing so with students having access to a computer.
The point of the problem I gave was not to find the derivative or even to know if a function is differentiable. The point is to test their understanding of the definition of derivative. It's a good problem in that is tests whether a student understands that in the two variable expression we call the difference quotient one of the variables is fixed for purposes of the definition. The students need to know that it is h that varies and not x. This is an unusual occurrence and requires some getting used to.
