=(900+1)/3*(900-9)/3+1/9
=(300+1/3)*(300-3)+1/9
=300*300+100-900-1+1/9
=90000-800-1+1/9
=89199又1/9题看错了~是90又三分之一乘以89又三分之二再加上九分之一=(90+1/3) *(90-1/3)+1/9=90*90-1/9+1/9平方差=8100