100乘90
(100+1)(90-1)=9000+90-100-1
=9000+9
=9009
=(100+1)×89
=100×89+1×890
=8900+89
=8989
=（101－1）?89=100x89+1x89=8900+1x89=8900+89=8989
简便计算101乘89
101*89=（100+1）*89=100*89+1*89=8900+89=8989（乘法分配律）
100乘90
(100+1)(90-1)=9000+90-100-1
=9000+9
=9009
=(100+1)×89
=100×89+1×890
=8900+89
=8989
=（101－1）?89=100x89+1x89=8900+1x89=8900+89=8989
100乘90
(100+1)(90-1)=9000+90-100-1
=9000+9
=9009
=(100+1)×89
=100×89+1×890
=8900+89
=8989
=（101－1）?89=100x89+1x89=8900+1x89=8900+89=8989