（99又1/2）^2
=99又1/2*99又1/2
=99又1/2*(100-1/2)
=(99+1/2)*100-(99+1/2)*1/2
=99*100+1/2*100-99*1/2-1/2*1/2
=9900+(1/2*100-99*1/2)-1/2*1/2
=9900+1/2*(100-99)-1/4
=9900+1/2-1/4
=9900又1/4