题目内容
设Sn=
+
+…+
则S10=______．
| 1 |
| 1×4 |
| 1 |
| 4×7 |
| 1 |
| (3n-2)(3n+1) |
∵Sn=
+
+…+
=
(1-
+
-
+…+
-
)
=
(1-
)
∴s10=
(1-
)=
故答案为：
| 1 |
| 1×4 |
| 1 |
| 4×7 |
| 1 |
| (3n-2)(3n+1) |
=
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| 4 |
| 1 |
| 7 |
| 1 |
| 3n-2 |
| 1 |
| 3n+1 |
=
| 1 |
| 3 |
| 1 |
| 3n+1 |
∴s10=
| 1 |
| 3 |
| 1 |
| 31 |
| 10 |
| 31 |
故答案为：
| 10 |
| 31 |
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