Sn^2=an(Sn-0.5)
=(Sn-Sn-1)(Sn-0.5)
Sn*Sn-1=0.5(Sn-1-Sn)
1/Sn-1/Sn-1=2
所以 1/Sn是以1为首项.公差为2的等差数列
Sn=1/2n-1