an+1-an=2,{an}是d=2的等差,
a9=a1+8d=17,所以 a1=1
an=a1+(n-1)d=2n-1
bn=3^an,b(n+1)=3^a(n+1),
b(n+1)/bn=3^[a(n+1)-an]=3^2=9,所以 {bn}是q=9的等比数列.
b1=3,
Sn=b1(1-q^n)/(1-q)=(3/8)(9^n -1)