For example, f'(2)=0, which means that f(x) has an extreme value when x=2.
There is something wrong with the above answer.
It can only be said that it may be an extreme point, such as f (x) = x 3.
F'(0)=0, but x=0 is not the extreme point of the function.
On the other hand, if the function is the extreme point of an x value, the derivative must be 0.