Sn=p^n
S(n-1)=p^(n-1)
S(n+1)=p^(n+1)
S(n-1)S(n+1)=p^(n-1)p^(n+1)=p^2n
(Sn)^2=p^2n
(Sn)^2=S(n-1)S(n+1)
数列{an}是等比数列