an=2/[(2n-1)(2n+1)]=1/(2n-1)-1/(2n+1),
∴数列{an}的前n项和
Sn=1-1/3+1/3-1/5+……+1/(2n-1)-1/(2n+1)
=1-1/（2n+1)
=2n/(2n+1).