| 4 |
| 13 |
| 4 |
| 3×4+1 |
| 7 |
| 22 |
| 7 |
| 3×7+1 |
又∵
| n |
| 3n+1 |
| 1 |
| 3 |
| 1 |
| 9n+3 |
∴当n=5，即
| n |
| 3n+1 |
| 5 |
| 16 |
| 4 |
| 13 |
| 5 |
| 16 |
| 7 |
| 22 |
当n=6时，即
| n |
| 3n+1 |
| 6 |
| 19 |
| 4 |
| 13 |
| 6 |
| 19 |
| 7 |
| 22 |
∴当b最小时，a+b=21，
故答案为21．
| 4 |
| 13 |
| a |
| b |
| 7 |
| 22 |
| 4 |
| 13 |
| 4 |
| 3×4+1 |
| 7 |
| 22 |
| 7 |
| 3×7+1 |
| n |
| 3n+1 |
| 1 |
| 3 |
| 1 |
| 9n+3 |
| n |
| 3n+1 |
| 5 |
| 16 |
| 4 |
| 13 |
| 5 |
| 16 |
| 7 |
| 22 |
| n |
| 3n+1 |
| 6 |
| 19 |
| 4 |
| 13 |
| 6 |
| 19 |
| 7 |
| 22 |