证明{xn}单调有界即可对于单调性x(n+1)-xn=1/(2n+1)+1/(2n+2)-1/(n+1)=1/[(2n+1)(2n+2)]>0，{xn}单增对于{xn}有界，下界显然xn>0而上界xn=1/(n+1)+1/(n+2)+...+1/(n+n)<1/n+1/n+...+1/n=1证毕
。。。只证不求的话。。
