题目内容
| 25.6×0.72+0.744×72; |
|
分析:(1)可将25.6×0.72变为0.256×72后根据乘法分配律计算.
(2)由可将原式变为
+
+
+…+
后,根据根据巧算公式:
=
-
计算.
(2)由可将原式变为
| 1 |
| 1×2 |
| 1 |
| 2×3 |
| 1 |
| 3×4 |
| 1 |
| 19×20 |
| 1 |
| n×(n+1) |
| 1 |
| n |
| 1 |
| n+1 |
解答:解:(1)25.6×0.72+0.744×72
=0.256×72+0.744×72
=(0.256+0.744)×72
=1×72
=72;
(2)
+
+
+
+…+
=
+
+
+…+
=1-
+
-
+
-
+…+
-
=1-
=
=0.256×72+0.744×72
=(0.256+0.744)×72
=1×72
=72;
(2)
| 1 |
| 2 |
| 1 |
| 6 |
| 1 |
| 12 |
| 1 |
| 20 |
| 1 |
| 380 |
=
| 1 |
| 1×2 |
| 1 |
| 2×3 |
| 1 |
| 3×4 |
| 1 |
| 19×20 |
=1-
| 1 |
| 2 |
| 1 |
| 2 |
| 1 |
| 3 |
| 1 |
| 3 |
| 1 |
| 4 |
| 1 |
| 19 |
| 1 |
| 20 |
=1-
| 1 |
| 20 |
=
| 19 |
| 20 |
点评:巧算公式:
=
-
在分数的巧算中经常用到,用熟练运用.
| 1 |
| n×(n+1) |
| 1 |
| n |
| 1 |
| n+1 |
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