题目内容
要使算式
+
+
+
+
+
+
+
+
+□=1成立，则□中的数等于
．
| 1 |
| 2 |
| 1 |
| 6 |
| 1 |
| 12 |
| 1 |
| 20 |
| 1 |
| 30 |
| 1 |
| 42 |
| 1 |
| 56 |
| 1 |
| 72 |
| 1 |
| 90 |
| 1 |
| 10 |
| 1 |
| 10 |
分析：因为
+
+
+
+
+
+
+
+
+□=1，所以□=1-（
+
+
+
+
+
+
+
+
），括号内的分数可以简算．
| 1 |
| 2 |
| 1 |
| 6 |
| 1 |
| 12 |
| 1 |
| 20 |
| 1 |
| 30 |
| 1 |
| 42 |
| 1 |
| 56 |
| 1 |
| 72 |
| 1 |
| 90 |
| 1 |
| 2 |
| 1 |
| 6 |
| 1 |
| 12 |
| 1 |
| 20 |
| 1 |
| 30 |
| 1 |
| 42 |
| 1 |
| 56 |
| 1 |
| 72 |
| 1 |
| 90 |
解答：解：
+
+
+
+
+
+
+
+
，
=
+
-
+
-
+…+
-
，
=1-
，
=
．
□=1-
，
=
．
| 1 |
| 2 |
| 1 |
| 6 |
| 1 |
| 12 |
| 1 |
| 20 |
| 1 |
| 30 |
| 1 |
| 42 |
| 1 |
| 56 |
| 1 |
| 72 |
| 1 |
| 90 |
=
| 1 |
| 2 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| 9 |
| 1 |
| 10 |
=1-
| 1 |
| 10 |
=
| 9 |
| 10 |
□=1-
| 9 |
| 10 |
=
| 1 |
| 10 |
点评：此题重点考查学生简算的能力．
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