则
| lim |
| h→0 |
| f(x0+h)-f(x0-h) |
| h |
=
| lim |
| h→0 |
| f(x0+h)-f(x0)+f(x0)-f(x0-h) |
| h |
=
| lim |
| h→0 |
| f(x0+h)-f(x0) |
| h |
| lim |
| -h→0 |
| f(x0-h)-f(x0) |
| -h |
=2f′（x0）=-6．
故选；B．
| lim |
| h→0 |
| f(x0+h)-f(x0-h) |
| h |
| lim |
| h→0 |
| f(x0+h)-f(x0-h) |
| h |
| lim |
| h→0 |
| f(x0+h)-f(x0)+f(x0)-f(x0-h) |
| h |
| lim |
| h→0 |
| f(x0+h)-f(x0) |
| h |
| lim |
| -h→0 |
| f(x0-h)-f(x0) |
| -h |