A1=S1=1/3(A1-1)
3A1=A1-1
A1=-1/2
S2=A1+A2=1/3(A2-1)
-3/2+3A2=A2-1
A2=-1/4求证数列an为等比数列Sn-S(n-1)=an所以1/3(an-1)-1/3[a(n-1)-1]=anan-a(n-1)=3an2an=-a(n-1)an/a(n-1)=-1/2所以是等比