数学递等式巧算题10.2X9.8 4.8X0.27除以3.2 (12.4+8.6)除以2.5X4 6.4X(1.25+2

10.2×9.8
=(10+0.2)×9.8
=10×9.8+0.2×9.8
=98+1.96
=99.96
4.8X0.27÷3.2
=4.8÷3.2×0.27
=1.5×0.27
=(1+0.5)×0.27
=0.27+0.5×0.27
=0.27+0.135
=0.405
(12.4+8.6)÷2.5X4
=21÷2.5×4
=21×4÷2.5
=84÷2.5
=33.6
6.4X(1.25+2.5-0.05)
=6.4X(1.25-0.05+2.5)
=6.4X(1.2+2.5)
=6.4X1.2+6.4X2.5
=6.4+6.4X0.2+1.6X4x2.5
=6.4+1.28+1.6X10
=6.4+1.28+16
=23.68
(90.2-9.17)÷3.7
=81.03÷3.7
=21.9
回答者：四七班9江户川

一：99.96 二：0.405 三:33.6 四：23.68 五:21.9

10.2X9.8 =(10+0.2)X(10-0.2)=10^2-0.2^2=100-0.04=99.96
4.8X0.27/3.2=(1.6X3)X0.27/(1.6X2)=3X0.27/2=0.405
(12.4+8.6)/2.5X4=21X0.4X4=8.4X4=33.6
6.4X(1.25+2.5-0.05)=8X0.8X1.25+16X0.4X2.5-3.2X2X0.05=8X10+16X10-3.2X0.1=80+160-0.32=239.68

10.2X9.8 =（10+0.2）（10-0.2）=10^2-0.2^2=100-0.04=99.96

好吧……就先一题吧
10.2X9.8=（10+0.2）*（10-0.2）=10²－0.2²=99.96