Question

已知数列{an}的前n项和为Sn,且Sn=1/3(an-1)(n∈正整数),求a1a2

已知数列{an}的前n项和为Sn,且Sn=1/3(an-1)(n∈正整数)
1、求a1，a2
2、求证：数列{an}是等比数列
要详细过程，拜托了

Answer 1

1. a1=S1 = 1/3 (a1 -1 ),a1 = -1/2
a1+a2 = S2 = 1/3(a2-1), a2=1/4
2. Sn=1/3(an-1),S(n-1)=1/3[a(n-1)-1)]
an = Sn - S(n-1) = 1/3(an-1)-1/3[a(n-1)-1)]
an = -1/2 a(n-1)
所以是等比数列，公比是 -1/2，首项是1/2

Answer 2

sn是多少啊？？？