口算
5
9
×15=
3
5
×2=
1
4
×
4
5
=
1
9
×
3
7
=
7
12
×
3
4
=
3
20
=
2
5
×
15
4
=
4
17
×34=
8
21
×
7
16
=
1
20
×
2
3
=
5
6
×
2
15
=
15
17
×
17
15
=
3
4
×
1
5
=
6
11
×
1
12
=
3
22
×
11
13
=
7
6
×24=
4
7
×
3
8
=
13
15
×30=
28×
7
24
=
5
6
×
3
7
=
分析:此題分為兩種類型:一種是分數(shù)與整數(shù)相乘,把分子與整數(shù)相乘的積做分子,分母不變,能約分的要約分;一種是分數(shù)與分數(shù)相乘,把分子相乘的積做分子,分母相乘的積做分母,能約分的要約分.
解答:解:
5
9
×15=
25
3
;        
3
5
×2=
6
5
;          
1
4
×
4
5
=
1
5
;         
1
9
×
3
7
=
1
21
;          
7
12
×
3
4
=
7
16

3
20
=
3
20
;            
2
5
×
15
4
=
3
2
;         
4
17
×34=8;         
8
21
×
7
16
=
1
6
;         
1
20
×
2
3
=
1
30
;
5
6
×
2
15
=
1
9
;           
15
17
×
17
15
=1;        
3
4
×
1
5
=
3
20
;        
6
11
×
1
12
=
1
22
;        
3
22
×
11
13
=
3
26

7
6
×24=28;            
4
7
×
3
8
=
3
14
;        
13
15
×30=26;        28×
7
24
=
49
6
;        
5
6
×
3
7
=
5
14
點評:此題考查了分數(shù)乘法的計算,掌握方法,注意約分.
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