Because in the field of x0, all x always have f(x)≤f(x0), that is, f(x0) is the largest. When △ x > 0 When f(x0+△x)-f(x0)≤0, dividing by △x is also ≤ 0; When △ x

Because it is in the domain of x0, that is, [x0-△x, x0+△x], the left and right domains of x0 are considered when discussing the positive and negative of △x.