题干
先化简，再求值：（1+
）÷
，其中a是小于3的正整数．
| 3 |
| a |
| 2 |
| a |
| + |
| 1 |
| a |
| 2 |
| 4 |
| 3 |
| a |
| 2 |
| a |
| + |
| 1 |
| a |
| 2 |
| 4 |
