How to judge the monotonicity of function with derivative
When f '(x) = 0, the extremum is the maximum. When f' (x) = 0, the extremum is the maximum. When f '(x) = 0, the extremum is the minimum. When f' (x) = 0, the extremum is the maximum. When f '(x) = 0, the extremum is the maximum, because a point is not monotonous