简便运算 99.9乘99+99.9

法一
=(100-0.1)*(100-1)+99.9
=10000-100-10+0.1+99.9
=10000-10
=9990
法二
=(100-0.1)*99+99.9
=9900+99.9-9.9
=9900+90
=9990
希望回答令你满意哦