| f '(x)-f '(x0)| & lt； ε,
Formula (1)
What does this expression mean, for? ε& gt； 0,? δ, when | x-x0 |
| f '(ξ)-f '(x0)| & lt； ε,
(2) and formula
What does this expression mean, for? ε& gt； 0,? Delta, when | ξ-x0 |
| f '(ξ)-f '(x0)| & lt； ε,
In this way, the difference comes out. (1) shows that no matter how small x is, there is always ξ | f' (ξ)-f' (x0) | < ε, and other points are not clear except ξ. ξ may be a series of points especially inclined to x0, which does not mean that f'(x) is continuous at x0. (2) Explain that when zeta is small enough, there is | f' (zeta)-f' (x0) | < ε, that is, a zeta 0 makes | f' (zeta 0)-f' (x0 )|.