第4课时乘法运算定律（1）运算定律3优翼www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com同学们你们知道每年的植树节是几月几日吗？探索新知一5 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com负责挖坑、种树的一共有多少人？25×4=100(人)4×25=100(人)我是这样计算的。我这样算也可以。探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com一共有25个小组，每组里4人负责挖坑、种树。负责挖坑、种树的一共有多少人？25×4＝1004×25＝10025×44×25＝你能再举几个这样的例子吗？探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com18×7○7×18124×35○35×124上面的每组算式有什么共同点？从上面的算式，可以发现什么规律？①每组算式中有两个因数，而且两个因数相同，只是交换了位置。②每个等式中，左右两边的因数的乘积相等。探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com5×4＝4×536×84＝84×36158×68＝68×158两个因数交换位置，积不变，这叫做乘法交换律。探索新知一a×b＝b×a如果用字母a、b表示两个因数，则可以写成： www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com65×145＝＿＿×＿＿109×31＝＿＿×＿＿44×98＝＿＿×＿＿346×273＝＿＿×＿＿用乘法交换律填上合适的数。14565311099844273346探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com一共要浇多少桶水？探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com一共要浇多少桶水？（25×5）×2=125×2=25025×（5×2）=25×10=250（25×5）×225×（5×2）=6探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com请你再举几个这样的例子。（3×6）×5=3×（6×5）7×（4×20）=（7×4）×20探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com从上面的算式中，你发现了什么？这叫做乘法结合律。三个数相乘，先乘前两个数或者先乘后两个数，积不变。探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com(a×b)×c=a×(b×c)能用a、b、c三个字母表示乘法结合律吗？用字母公式怎样表示？探索新知一 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com把应用乘法交换律的算式连起来。13×12c×bb×c2×5×72×7×513×12a×00×a巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com填空1、交换两个（）的位置，积（），这叫做乘法交换律，用字母表示是（）。2、75×26=（）×（）3、a×()=15×()因数不变a×b＝b×a267515a巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com6、一个数和1相乘得（）。7、一个数和0相乘得（）。8、23×25×4=4×()×()9、5×12×8=（）×8×（）这个数02523512巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com根据乘法运算定律填上合适的数。12×32=32×，108×75=×.30×6×7=30×（6×），125×（8×40）=（×）×.12751087125840巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com填空35×2×5=35×（2×）（60×25）×4=60×（×4）（125×5）×8=（×）×5（3×4）×5×6=（×）×（×）52512584536巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com利用发现的规律，计算。25×17×4（25×125）×（8×4）38×125×8×3巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com25×17×4=（25×4）×17=100×17=1700巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com（25×125）×（8×4）=（25×4）×（125×8）=100×1000=100000巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com38×125×8×3=（125×8）×（38×3）=1000×114=114000巩固练习二 www.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.comwww.youyi100.com两个数相乘，交换两个因数的位置，积不变，这叫做乘法交换律，用字母表示为a×b=b×a。三个数相乘，先乘前两个数，或者先乘后两个数，积不变，这叫做乘法结合律。用字母表示为：（a×b）×c=a×（b×c）课堂小结三