已知数列{an}的前n项和为Sn,a1=1,Sn=2a(n+1),则Sn=

Sn=2a(n+1)
= 2(S(n+1)-Sn)
2S(n+1) =3Sn
Sn/S(n-1) = 3/2
Sn/S1 = (3/2)^(n-1)
Sn = (3/2)^(n-1)
an = Sn - S(n-1)
= (1/2)(3/2)^(n-2)
= 3 .(3/2)^(n-1)
an = 3 .(3/2)^(n-1) ; n>=2
=1 ; n=1
再问： 那Sn=?
再答： Sn = (3/2)^(n-1)