题目内容
计算:
+
+
+
+…+
+
.
3 |
2 |
11 |
6 |
23 |
12 |
39 |
20 |
759 |
380 |
839 |
420 |
考点:分数的巧算
专题:计算问题(巧算速算)
分析:通过计算发现:每一项的结果都是“2-分数单位”的形式,分母为原来的分母.然后把分数拆分,通过加减相互抵消,即可求出结果.
解答:
解:
+
+
+
+…+
+
=(2-
)+(2-
)+(2-
)+…+(2-
)+(2-
)
=(2-
)+(2-
)+(2-
)+…+(2-
)+(2-
)
=2×19-(
+
+
+…
+
)
=38-[1-
+
-
+
-
+…
-
]
=38-(1-
)
=37+
=37
3 |
2 |
11 |
6 |
23 |
12 |
39 |
20 |
759 |
380 |
839 |
420 |
=(2-
1 |
2 |
1 |
6 |
1 |
12 |
1 |
380 |
1 |
420 |
=(2-
1 |
1×2 |
1 |
2×3 |
1 |
3×4 |
1 |
19×20 |
1 |
20×21 |
=2×19-(
1 |
1×2 |
1 |
2×3 |
1 |
3×4 |
1 |
19×20 |
1 |
20×21 |
=38-[1-
1 |
2 |
1 |
2 |
1 |
3 |
1 |
3 |
1 |
4 |
1 |
20 |
1 |
21 |
=38-(1-
1 |
21 |
=37+
1 |
21 |
=37
1 |
37 |
点评:此题解答的关键在于把分数拆分,变成相互抵消的形式,使计算简便.
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