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加法运算定律运算定律人教版四年级数学下册
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www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com根据所给的条件,你能提出什么数学问题吗?1探索新知二
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www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com观察每组算式等号两边有什么相同点和不同点?数没变,符号没有变,只是加数位置发生了变化。问题:1.像这样的算式你写的完么?探索新知二
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com请大家用自己喜欢的方式表示加法交换律。问题:2.这些算式有什么共同的特点?这就叫做加法交换律。两个数相加,交换加数的位置,和不变。探索新知二
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com通常我们数学上可以用字母表示数。今天我们就选字母a和b来表示两个加数。a表示第一个加数,b表示第二个加数。用字母就可以表示成:a+b=b+a比如:+=+探索新知二
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com这里有三组算式,在○里填上适当的符号。(12+13)+14○12+(13+14)(30+28)+60○30+(28+60)(320+150)+230○320+(150+230)观察这三个等式,它们有什么相同的地方?===探索新知二
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com等号左右两边的算式在运算顺序上有什么不同?但它们的结果怎样?从以上问题你发现了什么规律?(12+13)+14○12+(13+14)(30+28)+60○30+(28+60)(320+150)+230○320+(150+230)===大家发现的这个规律我们把它叫做加法结合律。探索新知二
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www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com这三天一共骑了多少路程?88+104+962探索新知二
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com怎样计算比较简便?要应用什么运算定律?方法188+104+96=192+96=288探索新知二
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com方法288+104+96=88+(104+96)=88+200=288→应用加法结合律上边两种方法在应用运算定律方面有什么不同?第一道是按从左到右的顺序计算,而第二道应用了加法结合律。探索新知二
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www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com3578+=43+1.根据加法交换律填空。3004378a300+600=600++65=65+35a+12=12+巩固练习三
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com2.应用加法交换律,用线连一连。28+5679+O۞+69۞+O56+28O+7969+۞O+۞巩固练习三
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com3.应用加法交换律在下面填上适当的数。29+17=+29128+=15++=323+18654+x=+1712815323186x54巩固练习三
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com4.填空。(1)一个数加0,还得()。(2)两个加数()位置,()不变,这叫做加法()。交换和交换律这个数巩固练习三
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com5.下面各等式哪些符合加法交换律?符合的画“√”。(1)276+124=180+220()(2)a+20=400+a()(3)550+240=240+550()(4)a+c=c+a()√√巩固练习三
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www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com7.学生先思考,然后指名回答,并说出错的原因。下面各题计算中应用了什么运算定律:(1)283+152+48=283+(152+48)=283+200=483加法结合律巩固练习三
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com(2)154+87+246+13=154+246+87+13=(154+246)+(87+13)=400+100=500先应用了加法交换律,又应用了加法结合律。巩固练习三
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com两个数相加,交换加数的位置,和不变,这叫做加法交换律。用字母可以表示为:a+b=b+a三个数相加,先把前两个数相加,或者先把后两个数相加,和不变,这叫做加法结合律。用字母表示为:(a+b)+c=a+(b+c)。课堂小结四
www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com课后练习五
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