Let f(x) be a real-valued function and let c be a number in the domain of f(x). If f'(c) = 0 or f'(c) does not exist, then c is a critical number of f(x). If c is a critical number of f(x), the value f(c) is called a critical value of f(x) and the point (c, f(c)) is called a critical point of f(x).