数学教学课件第七册第四单元制作:吴金志单击鼠标操作1.视算0.2x0.5=0.10.27-0.2=0.070.8x0.7=0.564.5x3=13.51.5x0.4=0.60.05x1.6=0.087.2+2.8=100.93x100=930.06x0.9=0.0540.1x6.1=0.611.2-0.8=0.40.75+0.3=1.052.用简便方法计算下面各题25x93x452x8x12548x12575x10168x7+32x7应用什么计算定律使计算简便?=25x4x93=100x93=9300=52x1000=52000=6x8x125=6x1000=6000=75x(100+1)=75x100+75=7500+75=7575=(68+32)x7=100x7=7003.尝试计算0.25x4.78x40.65x201=0.25x4x4.78=1x4.78=4.78用乘法交换律把算式改写成0.25x4x4.78.因为0.25乘4等于1,这样算简便=0.65x(200+1)=0.65x200+0.65x1=130+0.65=130.65把201看成(200+1),用乘法分配律把0.65乘200加0.65乘1,这样算简便.4.练一练,用简便方法计算下列各题0.034x0.5x0.6102x0.45=0.034x0.3=0.0102=(100+2)x0.45=100x0.45+2x0.45=45+0.9=45.9讨论:小数乘法怎样算简便?如果使一个因数转化成整千整百整十---的数或者使两个因数的乘积为整千整百---的数能使计算简便.5.巩固练习:说说下列各题怎样算简便0.3x2.5x0.40.78x1011.2x2.5+0.8x2.550x0.13x0.21.25x0.7x0.80.8x630x0.1250.038x5x2.19.8x25=0.038x10.5=0.038x(10+0.5)=0.038x10+0.038x0.5=0.38+0.019=0.399=25x9.8=25x10-25x0.2=250-5=245作业:课本95页练习22678
