where we assume that h > 0. What do we mean when we say that the expression on
the right-hand-side of (5.1) is an approximation of the derivative? For linear functions
(5.1) is actually an exact expression for the derivative. For almost all other functions,
(5.1) is not the exact derivative.
Let’s compute the approximation error. We write a Taylor expansion of f(x + h)
about x, i.e.,