设x∈(2,4),则1/5<1/(1+x)<1/3
对任意的ε>0,取 δ= min{1,3ε}则当0<|x-3|<δ时,
|(x-3)/(x+1)-0|<(1/3)|x-3|<ε,
即lim x-3)/(x+1)=0