1=(18.7-8.7)x28=10x28=280
2=1.25x8+2.5x8=10+20=30
3 =101x(101-1)=101x100=10100
4 打错(57应为.57吧)结合律
5=0.25x(76-46+70)=0.25x100=25
6 =5/8x(99+1)=5/8x100=125/2
7 =5/7x3/7+4/7x5/7=5/7x(3/7+4/7)=5/7x1=5/7
8 =(9.9+0.1)+(99.9+0.1)+(999.9+0.1)=10+100+1000=1110