Definition, f is a function that find the derivative on the interval (a, b) values of c (a, b), which makes the f ' (c) = 0 are called critical values (critical value) of the function f.
Definition Let f be a differentiable function on the interval (a, b) values? c (a, b), which gives f '(c) = 0 are called critical value (critical value) of the function f.