?$\frac {2}{5}=0.4,$??$\frac {7}{2}=3.5,$?
因?yàn)?$0.4<2<3.5,$?所以?$\frac {2}{5}<2<\frac {7}{2}。$?
?$ $?它們的倒數(shù)分別為?$\frac {5}{2}=2.5,$??$\frac {2}{7}≈0.29,$??$\frac {1}{2}=0.5,$?
?$ $?因?yàn)?$0.29<0.5<2.5,$?所以?$\frac {2}{7}<\frac {1}{2}<\frac {5}{2}。$?