a1=1,由上面得：Sn-Sn1+1/（Sn-Sn-1）=2Sn,化简得：Sn*Sn-Sn-1*Sn-1=1,得到一个等差数列,求出Sn*Sn=n,则an=根号n-根号（n-1)