整数乘法运算定律推广到小数周道敏2013年9月24日1、口算（直接写出得数）。(1)48+52=4.8+5.2=(2)38+62=0.38+0.62=(3)4×25=0.4×2.5=（4）125×8=1.25×8=1001001001011101000复习引入2、能用简便方法计算下列各题吗？（1）25×36×4（2）16×125×8（3）92×46+92×54•男生队男生队1.25×0.98×0.81.25×0.98×0.80.8×0.4×0.50.8×0.4×0.52.4×0.5+3.6×0.52.4×0.5+3.6×0.5•女生队女生队1.25×0.8×0.981.25×0.8×0.980.8×0.8×（（0.4×0.50.4×0.5））（（2.4+3.62.4+3.6））×0.5×0.5探究新知•女生队女生队1.25×0.98×0.81.25×0.98×0.80.8×0.4×0.50.8×0.4×0.52.4×0.5+3.6×0.52.4×0.5+3.6×0.5•男生队男生队1.25×0.8×0.981.25×0.8×0.980.8×0.8×（（0.4×0.50.4×0.5））（（2.4+3.62.4+3.6））×0.5×0.51.25×0.98×0.81.25×0.98×0.8=12.25×0.8=12.25×0.8=0.98=0.980.8×0.4×0.50.8×0.4×0.5=0.32×0.5=0.32×0.5=0.16=0.162.4×0.5+3.6×0.52.4×0.5+3.6×0.5=1.2+1.8=1.2+1.8=3=31.25×0.8×0.981.25×0.8×0.98=1×0.98=1×0.98=0.98=0.980.8×0.8×（（0.4×0.50.4×0.5））=0.8×0.2=0.8×0.2=0.16=0.16（（2.4+3.62.4+3.6））×0.5×0.5=6×0.5=6×0.5=3=3===比一比1.25×0.98×0.8=1.25×0.8×0.981.25×0.98×0.8=1.25×0.8×0.98说一说乘法交换律乘法交换律a×b=b×aa×b=b×a0.8×0.4×0.5=0.8×0.8×0.4×0.5=0.8×（（0.4×0.50.4×0.5））乘法结合律乘法结合律a×b×c=a×b×ca×b×c=a×b×c2.4×0.5+3.6×0.5=2.4×0.5+3.6×0.5=（（2.4+3.62.4+3.6））×0.5×0.5乘法分配律乘法分配律a×c+b×c=a×c+b×c=（（a+ba+b））×c×c例题分析1、1.25×0.73×42、0.32×1021、0.25×0.73×4=0.25×4×0.73=1×0.73=0.73乘法交换律2、0.32×102=0.32×（100+2）=0.32×100+0.32×2=32+0.64=32.64乘法分配律怎样计算简便就怎样计算（说说我是怎样想的，理由是什么,计算时应注意什么）。(1)0.25×32(2)4.9×99+4.6(3)0.25×101－0.25课堂练习动脑筋一你能用几种简便方法计算呢？（1）0.25×16（2）12.5×8.8动脑筋二2.3×86+77×8.6归纳：在同一个乘法算式中，将一个因数扩大若干倍，另一个因数缩小了相同的倍数，积不变。这样的规律也可以运用到小数乘法的简便计算中。小结与复习作业11.11×6666+77.78×3333
