If you take the maximum value on a or b, their derivatives are 0, that is, if you take the maximum value on point A, the derivative of point A is 0, the right derivative of the point with the maximum value is less than 0, and the left derivative is greater than 0, which are the two known antonyms in the question. But if you know that point A takes the maximum value, you know that it does not meet the two known opposites, so the maximum value obtained is between ab, that is, the extreme derivative is 0. Hope to adopt, thank you.