31x99=31*(100-1)=31*100-31=3100-31=3069
125*81 =125x(80+1)=125x80+125x1=10000+125=1012531x99=31x(100-1)=31x100-31x1=3100-31=3069 83+99x83=(1+99)x83=100x83=8300
31X99=31X(100-1)=3100-31=306942X98=42X(100-2)=4200-84=411655X19+55=55X(19+1)=55X20=1100 望采纳 有不懂的地方欢迎追问
31*99=31*(100-1)=31*100-31*1=3100-31=306931*99=31*(90+9)=31*90+31*9=2790+279=3069
36*99=36*(100-1)=36*100-36*1=3600-36=3564
33x99+33=33x(99+1)=33x100=3300
56+56x99=56x(1+99)=56x100=5600 得到的结果为5600
139*99=139*(100-1)=139*100-139=13900-139=13761
31 * 99 = 31 * (100 -1) = 33 * 100 - 33 * 1 = 3300 - 33 = 3267