题目内容
求和：
+
+…+
=______．
| 1 |
| 1×4 |
| 1 |
| 4×7 |
| 1 |
| (3n-2)×(3n+1) |
设Sn=
+
+…+
则3Sn=
+
+…+
=1-
+
-
+…+
-
=1-
=
所以Sn=
．
故答案为
| 1 |
| 1×4 |
| 1 |
| 4×7 |
| 1 |
| (3n-2)×(3n+1) |
则3Sn=
| 3 |
| 1×4 |
| 3 |
| 4×7 |
| 3 |
| (3n-2)×(3n+1) |
| 1 |
| 4 |
| 1 |
| 4 |
| 1 |
| 7 |
| 1 |
| (3n-2) |
| 1 |
| (3n+1) |
| 1 |
| (3n+1) |
| 3n |
| (3n+1) |
所以Sn=
| n |
| (3n+1) |
故答案为
| n |
| (3n+1) |
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