问题 解答题
下面各题怎样算简便怎样算.
(1)3643-74+6357-76
(2)19.99÷2
1
2
-0.4×9.99
(3)8500÷25
(4)
1
2
+
1
6
+
1
12
+
1
20
+
1
30
+
1
42
+
1
56
+
1
72
答案

(1)3643-74+6357-76

=3643+6357-(74+76),

=10000-150,

=9850;

(2)19.99÷2

1
2
-0.4×9.99

=19.99×0.4-0.4×9.99

=(19.99-9.99)×0.4,

=10×0.4,

=4;

(3)8500÷25=340;

(4)

1
2
+
1
6
+
1
12
+
1
20
+
1
30
+
1
42
+
1
56
+
1
72

=

1
1×2
+
1
2×3
+…+
1
8×9

=1-

1
2
+
1
2
-
1
3
+…+
1
8
-
1
9

=1-

1
9

=

8
9

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