五上数学-小数乘除法简便运算专项

五上数学——小数乘除法简便运算专项一、乘法结合律4.36×12.5×8=4.36×(12.5×8)=4.36×100=4360.95×0.25×4=0.95×(0.25×4)=0.95×1=0.950.75×50×0.4=0.75×(50×0.4)=0.75×20=15二、乘法交换律0.25×8.5×4=0.25×4×8.5=1×8.5=8.512.5×0.96×0.8=12.5×0.8×0.96=10×0.96=9.60.5×0.34×0.6=0.5×0.6×0.34=0.3×0.34=0.102三、乘法分配律(125+2.5)×0.8=125×0.8+2.5×0.8=100+2=102(正用)(1.25-0.125)×8=1.25×8-0.125×8=10-1=9(正用)3.72×3.5+6.28×3.5=3.5×(3.72+6.28)=3.5×10=35(逆用)四、拆数+乘法结合律0.125×8.8=0.125×8×1.1=1×1.1=1.10.25×0.28=0.25×4×0.07=1×0.07=0.070.125×3.2×2.5=(0.125×8)×(0.4×2.5)=1×1=1五、除法的性质正用：a÷b÷c=a÷(b×c)320÷1.25÷8=320÷(1.25×8)=320÷10=323.52÷2.5÷0.4=3.52÷(2.5×0.4)=3.52÷1=3.529.6÷0.8÷4=9.6÷(0.8×4)=9.6÷3.2=3逆用：a÷(b×c)=a÷b÷c3.9÷(1.3×5)=3.9÷1.3÷5=3÷5=0.615÷(0.15×0.4)=15÷0.15÷0.4=100÷0.4=25075.3÷(7.53×20)=75.3÷7.53÷20=10÷20=0.5六、把其中一个因数分成两个数的和或差，再按乘法分配律0.8×100.1=0.8×(100+0.1)=0.8×100+0.8×0.1=80+0.08=80.083.65×10.1=3.65×(10+0.1)=3.65×10+3.65×0.1=36.5+0.365=36.8650.32×403=0.32×(400+3)=0.32×400+0.32×3=128+0.96=128.964.6×0.9=4.6×(1-0.1)=4.6×1-4.6×0.1=4.6-0.46=4.140.85×199=0.85×(200-1)=0.85×200-0.85×1=170-0.85=169.150.79×99=0.79×(100-1)=0.79×100-0.79×1=79-0.79=78.21七、添加因数“1”56.5×99+56.5=56.5×99+56.5×1=56.5×(99+1)=56.5×100=56509.7×99+9.7=9.7×99+9.7×1=9.7×(99+1)=9.7×100=9704.2×99+4.2=4.2×99+4.2×1=4.2×(99+1)=4.2×100=42012.7×9.9+1.27=1.27×99+1.27×1=1.27×(99+1)=1.27×100=1271.87×9.9+0.187=1.87×9.9+1.87×0.1=1.87×(9.9+0.1)=1.87×10=18.75.4×11-5.4=5.4×11-5.4×1=5.4×(11-1)=5.4×10=54八、更改因数的小数点位置6.66×3.3+0.666×67=6.66×3.3+6.66×6.7=6.66×(3.3+6.7)=6.66×10=66.645×57+23×86=46×57+46×43=46×(57+43)=46×100=46004.8×7.8+78×0.52=4.8×7.8+7.8×5.2=7.8×(4.8+5.2)=7.8×10=783.14×0.68+31.4×0.032=3.14×0.68+3.14×0.32=3