人教版2020年春四年级下册数学第3单元《第4课时 乘法运算定律》（1）

第4课时乘法运算定律1运算定律优翼wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com同学们你们知道每年的植树节是几月几日吗探索新知一5wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com负责挖坑种树的一共有多少人25×4100人4×25100人我是这样计算的我这样算也可以探索新知一wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com　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＝10025×4　　4×25＝你

能再举几个这样的例子吗探索新知一wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com18×7○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wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com5×4＝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ab表示两个因数则可以写成wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com65×145＝＿＿×＿＿109×31＝＿＿×＿＿44×98＝＿＿×＿＿346×273＝＿＿×＿＿用乘法交换律填上合适的数14565311099844273346探索

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探索新知一wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100coma×b×ca×b×c能用abc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wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com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wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com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275×26×3a×15×因数不变a×b＝b×a267515a巩固练习二wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouy

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wyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com25×17×425×4×17100×171700巩固练习二wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com25×125×8×425×4×125×8100×1000100000巩固练习二wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com38×125×8×3125×8×38×31000×114114000巩固练习二wwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100comwwwyouyi100com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×bb×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×ca×b×c课堂小结三