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神机妙算巧又对.
5
8
+
7
9
+
3
8
17
18
-
2
5
-
1
5
5
9
-
5
12
+
4
9
2
3
+
7
10
+
1
3
+
3
10
8
9
-(
1
5
-
1
9
5
12
+
11
15
-
6
15
+
7
12
分析:横向数:
(1)(3)运用加法交换律解答,
(2)(5)运用减法性质解答,
(4)(6)运用加法交换律和结合律解答.
解答:解:(1)
5
8
+
7
9
+
3
8

=
5
8
+
3
8
+
7
9

=1+
7
9

=1
7
9


(2)
17
18
-
2
5
-
1
5

=
17
18
-(
2
5
+
1
5
),
=
17
18
-
3
5

=
31
90


(3)
5
9
-
5
12
+
4
9

=
5
9
+
4
9
-
5
12

=1-
5
12

=
7
12


(4)
2
3
+
7
10
+
1
3
+
3
10

=(
2
3
+
1
3
)+(
7
10
+
3
10
),
=1+1,
=2;

(5)
8
9
-(
1
5
-
1
9
),
=
8
9
+
1
9
-
1
5

=1-
1
5

=
4
5


(6)
5
12
+
11
15
-
6
15
+
7
12

=(
5
12
+
7
12
)+(
11
15
-
6
15
),
=1+
1
3

=1
1
3
点评:本题主要考查学生:正确选择并运用简便算法,解决问题的能力.
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