用简便方法计算下面各题.(1)12.5×2.5×3.2(2)5.6÷0.32(3)100.1×23(4) 8 9 -

用简便方法计算下面各题.(1)12.5×2.5×3.2(2)5.6÷0.32(3)100.1×23(4) 8 9 - 1 2 + 5 8 + 1 2 (5)1- 5 12 - 7 12 (6) 7 18 -( 5 18 - 8 9 )


(1)12.5×2.5×3.2
=12.5×2.5×(0.4×8),
=(12.5×8)×(2.5×0.4),
=100×1,
=100;

(2)5.6÷0.32
=5.6÷(0.8×0.4),
=5.6÷0.8÷0.4,
=7÷0.4,
=17.5;

(3)100.1×23
=(100+0.1)×23,
=100×23+0.1×23,
=2300+2.3,
=2302.3;

(4)
8
9
-
1
2
+
5
8
+
1
2

=(
8
9
+
5
8
)+(
1
2
-
1
2
),
=
8
9
+
5
8

=
109
72


(5)1-
5
12
-
7
12

=1-(
5
12
+
7
12
),
=1-1,
=0;

(6)
7
18
-(
5
18
-
8
9

=
7
18
-
5
18
+
8
9

=
1
9
+
8
9

=1.
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