小数乘法
——计算法则
例题1：一个风筝3.5元，小明买3个，一共花了多少元？
分析：根据数量关系：单价×数量=总价
![](https://wkrtcs.bdimg.com/rtcs/image?w=216.00&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_6","w":"216.00","h":"112.00","dataType":"gif","c":"word/media/image1.gif"})
如何计算3.5×3呢？
方法一：分解组合法
![](https://wkrtcs.bdimg.com/rtcs/image?w=177.33&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_6","w":"177.33","h":"96.00","dataType":"gif","c":"word/media/image2.gif"})
方法二：转化法
知识点1：小数乘整数的意义和整数乘法的研究相同，都是几个相同的加数的和的简便运算。
例题2计算0.72×5![](https://wkrtcs.bdimg.com/rtcs/image?w=236.00&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_10","w":"236.00","h":"200.00","dataType":"gif","c":"word/media/image3.gif"})
0 . 7 2 7 2
![](https://wkrtcs.bdimg.com/rtcs/image?w=113.33&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_15","w":"113.33","h":"13.00","dataType":"gif","c":"word/media/image4.gif"})
3 . 6 0 3 6 0![](https://wkrtcs.bdimg.com/rtcs/image?w=105.33&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_7","w":"105.33","h":"13.00","dataType":"gif","c":"word/media/image5.gif"})
知识点2：小数乘整数的计算方法：先按照整数乘法计算，再看因数中一共有几位小数，就从积的右边起数几位，积的小数部分末位的0要去掉。
知识点3，小数的基本性质：小数的末尾添上0或去掉0，小数的大小不变。
练习：（用竖式计算）
1. 4.2×6= 8.1× 5= 0.85×14=
2.小美家距离学校2.7km，他每天早晨步行去学校，下午放学步行回家，小梅上学两天一共步行多少千米？
例题3,：计算1.92×0.9
1. 9 2 1 9 2![](https://wkrtcs.bdimg.com/rtcs/image?w=146.67&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_7","w":"146.67","h":"13.33","dataType":"gif","c":"word/media/image6.gif"})
× 0.9 × 9 ![](https://wkrtcs.bdimg.com/rtcs/image?w=146.67&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_16","w":"146.67","h":"13.33","dataType":"gif","c":"word/media/image7.gif"})
1. 7 2 8 1 7 2 8![](https://wkrtcs.bdimg.com/rtcs/image?w=142.67&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_7","w":"142.67","h":"13.33","dataType":"gif","c":"word/media/image8.gif"})
观察因数与积的小数位数关系：
两个因数的小数位数和等于积中的小数位数
知识点4：小数乘法的计算方法：
（1）按照整数乘法算出积；
（2）看因数中一共有几位小数，就从积的右边起数出几位，点上小数点；
（3）积的小数位数不够，要在前面用0补位；
（4）积的小数部分末尾有0要把0去掉
练习：
0.4×8= 4.78×4= 0.65×200=
2.6×0.57= 1.09×1.2= 4.2×5.15=
能力提升：解决数学谜问题
![](https://wkrtcs.bdimg.com/rtcs/image?w=213.93&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_5","w":"213.93","h":"589.00","dataType":"jpeg","c":"word/media/image9.jpeg","imgOriW":490,"imgOriH":1350})
练习：
![](https://wkrtcs.bdimg.com/rtcs/image?w=501.80&md5sum=264b7bcf3405d7d88993c9826734ace6&sign=1d851312c9&rtcs_flag=2&rtcs_ver=3.1&l=webapp&bucketNum=74&ipr={"t":"img","r":"r_5","w":"501.80","h":"269.53","dataType":"jpeg","c":"word/media/image10.jpeg","imgOriW":1428,"imgOriH":767})
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