题干
已知 f(x)=
， f(1)=
，f（2）=2，f[g（x）]=4﹣x．
| a |
| x |
| x |
| + |
| b |
| 5 |
| 4 |
| a |
| x |
| x |
| + |
| b |
| 5 |
| 4 |
