Taylor expansion expands x-r x+r at x, where r is an arbitrary constant satisfying x+r x-r.

Then you add and subtract the two formulas, move everything in f' to one side, and then add the absolute value sign to enlarge it. . . Publish a quadratic equation about r (M2 is not equal to 0 at this time) and the discriminant is less than or equal to 0 (just get what you want to prove).

If you can deduce the above things, then M2 is equal to 0, which is easier.