题目内容
求和：1+
+
+…+
．
| 4 |
| 5 |
| 7 |
| 52 |
| 3n-2 |
| 5n-1 |
设Sn=1+
+
+…+
+
①?
则
Sn=
+
+
+…+
+
②?
①-②得：?
Sn=1+
+
+…+
-
=1+3×
-
=
∴Sn=
| 4 |
| 5 |
| 7 |
| 52 |
| 3n-5 |
| 5n-2 |
| 3n-2 |
| 5n-1 |
则
| 1 |
| 5 |
| 1 |
| 5 |
| 4 |
| 52 |
| 7 |
| 53 |
| 3n-5 |
| 5n-1 |
| 3n-2 |
| 5n |
①-②得：?
| 4 |
| 5 |
| 3 |
| 5 |
| 3 |
| 52 |
| 3 |
| 5n-1 |
| 3n-2 |
| 5n |
=1+3×
| ||||
1-
|
| 3n-2 |
| 5n |
=
| 7×5n-12n-7 |
| 4×5n |
∴Sn=
| 7×5n-12n-7 |
| 16×5n-1 |
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