The derivative of a function f (x) at x = x0, denoted f'(x0) or (x0), can be naively defined as the slope of the graph of f at x = x0. The problem is that we have not said what we mean by the slope of an arbitrary graph at a point. We do, however, know what we mean by the slope of a line. Therefore, we define the slope of the graph of f at a point x0 to be the slope of the tangent line to the graph at x0. This tangent line can be thought of in a couple of ways:
In order for the tangent line to be well-defined, the graph of f at x0 must be sufficiently smooth. Furthermore, the tangent line must not be vertical, for a vertical line is not a function, and cannot be assigned a slope. If the slope of the tangent line, and hence the derivative of f, are well-defined at a point x0, we say f is differentiable at x0. As would be expected, a function that is differentiable at a point must also be continuous at that point. On the other hand, not all functions that are continuous at a point are also differentiable at that point. For example, consider the absolute value function at x = 0.