题干
若数列{an}是正项数列，且 a1 +a2 +⋯+an =n2 +3n ，则
+
+
+⋯+
=____．
| a |
| 1 |
| 2 |
| a |
| 2 |
| 3 |
| a |
| 3 |
| 4 |
| a |
| n |
| n |
| + |
| 1 |
答案(点此获取答案解析）
2n2+6n
