2Sn=3an-3, 2S(n-1) = 3a(n-1) -3
作差： 2[Sn -S(n-1)]= 3an - 3a(n-1)
2an = 3an -3a(n-1)
an = 3a(n-1)
an/ a(n-1) =3
数列{AN}是等比数列,公式是3,
当n=1时,a1=3a1-3,a1= 3/2
an= (3/2)* 3^(n-1) = 3^n/2