Give an example for the following

A function f that is continous at x = a, not differentiable at x = a, but yet f attains a relative extremum at x = a

Let f(x) = -x when x < 0, and = x when x >= 0. Thus, x is continuous at 0, not differentiable at 0 (there is a corner at 0), and f(0) = 0 is a minimum.