是做填空题的话,只要满足S0=0即可.求得a=-1.
要是解答题的话,可先求a1=S1=3a+1.
当n>1时,an=Sn-S(n-1)=a*3^n+1-[a*3^(n-1)+1]=2a*3^(n-1)
a1也满足上式,有3a+1=2a,所以a=-1.