题干
对于命题P：存在一个常数M，使得不等式
+
≤M≤
+
| a |
| 2 |
| a |
| + |
| b |
| b |
| 2 |
| b |
| + |
| a |
| a |
| a |
| + |
| 2 |
| b |
| b |
| b |
| + |
| 2 |
| a |
对任意正数a，b恒成立．
