如图，过D点作AB的平行线EF，交BC，AC于E，F．△MDE∽△MAB，因为DM=AD，所以DE=
| 1 |
| 2 |
又因为△CFE∽△CAB，所以
| FE |
| AB |
| CE |
| BC |
因为BM=3MC，所以
| CE |
| BC |
| 2.5 |
| 4 |
| 5 |
| 8 |
| 5 |
| 8 |
所以DF=EF-ED=
| 5 |
| 8 |
| 1 |
| 2 |
| 1 |
| 8 |
由△NFD∽△NAB得
| ND |
| NB |
| FD |
| AB |
| 1 |
| 8 |
如图，过D点作AB的平行线EF，交BC，AC于E，F．| 1 |
| 2 |
| FE |
| AB |
| CE |
| BC |
| CE |
| BC |
| 2.5 |
| 4 |
| 5 |
| 8 |
| 5 |
| 8 |
| 5 |
| 8 |
| 1 |
| 2 |
| 1 |
| 8 |
| ND |
| NB |
| FD |
| AB |
| 1 |
| 8 |