an=2*s²n/(2sn-1))
(sn-s(n-1))(2sn-1)=2s²n
2s²n-sn-2sns(n-1)+s(n-1)=2s²n
s(n-1)-sn=2sn*s(n-1)
1/sn-1/s(n-1)=2
s1=1,1/s1=1
1/Sn是以1为首项,公差为2的等差数列
1/sn=1+2(n-1)=2n-1
sn=1/(2n-1)
an=sn-s(n-1)
=1/(2n-1)-1/(2(n-1)-1)
=1/(2n-1)-1/(2n-3)
=(2n-3-2n+1)/(2n-1)(2n-3)
=-2/(4n²-7n+3)