1、乘、除法混合运算中的巧算,前备知识,混合运算中去掉(添加)括号的规律 如果括号前是除号，那么去掉括号后，括号内的除号变乘号，乘号变除号 如果括号前是乘号，去括号后则不需要改变括号内的乘号或除号 添加括号的规律也是这样,前备知识,a(bc)=abc a(bc)=abc a(bc)=abc,例1,(1)450(925) =450925 =5025 =2,(2)1280(644) =1280644 =204 =80,拓展1,(1)756(79) =75679 =1089 =12,(2)875000(10008) =87500010008 =8758 =71258 =71000 =7000,拓展1,(
2、3)1000(1254) =10001254 =84 =32,(4)125(82) =12582 =10002 =500,例2,(1)56047 =560(47) =56028 =20,(2)124168 =124(168) =1242 =248,拓展2,1200254 =1200(254) =1200100 =12,40001258 =4000(1258) =40001000 =4,拓展2,720124 =720(124) =7203 =2160,444728182 =444(728182) =4444 =1776,拓展3,52575 =52557 =1057 =15,25501725 =2
3、5502517 =10217 =6,拓展4,728(138) =728138 =728813 =9113 =7,810(159) =810159 =810915 =9015 =6,例3,48025 =480(1004) =4801004 =4804100 =120100 =12000,拓展5,12858 =12885 =165 =80,520125 =520(10008) =52010008 =52081000 =651000 =65000,总结,乘、除法计算中经常可以通过增加、去掉括号和改变数字的顺序来使计算简便 在乘除法混合运算中运用交换律时，运算符号要带着走。abc=acb=cba 可以将25看作1004，将125看作10008，让计算简化,作业：拓展6,123456789456789123 =123123456456789789 =111 =1,
