设等比数列{an}的公比为q,S1,2S2,3S3成等差数列，<==>4S2=S1+3S3,a1=1,<==>4(1+q)=1+3(1+q+q^2),∴3q^2-q=0,q≠0，∴q=1/3,∴an=1/3^(n-1).
