一：Sn=[n/(n+2)]*a(n+1)
Sn-S(n-1)=an=[n/(n+2)]*a(n+1)-[(n-1)/(n+1)]*an
[2n/(n+1)]*an=[n/(n+2)]*a(n+1)
[a(n+1)]/an=2[(n+2)/(n+1)]
又Sn/n=[a(n+1)]/(n+2)
(Sn/n)/(S(n-1)/(n-1))=2
数列｛sn/n｝成等比数列
第二问暂时没有出来,an=2的（n-2）次方*（n+1）