But after reading the answer, it is quite simple. Generally, this function topic of seeking increase or decrease is based on this idea. First, take the derivative, determine the region, and then compare the increased or decreased values. But it's strange that you said you couldn't get guidance. Where did you not get it? The derivative of the multiplication of the two methods is the first pilot multiplied by the second non-derivative, and then the product of the two numbers is added (the second pilot multiplied by the first non-derivative), have you forgotten here? Then according to the derivative formula of quadratic function, the derived function can be disassembled. Is that what you did? Next is the area to be increased or decreased. You know this, so do you understand what I mean? I'm worried about my expressive ability ... If you don't know, please keep asking, it doesn't matter.