题干
已知a，b，c为正实数，且a+b+c=3，证明：
+
+
≥3．
| c |
| 2 |
| a |
| a |
| 2 |
| b |
| b |
| 2 |
| c |
| c |
| 2 |
| a |
| a |
| 2 |
| b |
| b |
| 2 |
| c |
