你应该学过等比数列吧！当n→∞时，San=1+(1/2)+(1/2)²+……+(1/2)^(n-1)=1/[1-(1/2)]=2Sbn=1+(1/3)+(1/3)²+……+(1/3)^(n-1)=1/[1-(1/3)]=3/2所以，limXn=2/(3/2)=4/3
