S3=a1+a1q+a1q^2=a1(1+q+q^2)
a3=a1q^2
(1+q+q^2)/q^2=S3/a3=13
12q^2-q-1=0
(3q-1)(4q+1)=0
q=1/3
a1=a3/q^2=(1/27)/(1/9)=1/3
an=a1q^(n-1)=(1/3)(1/3)^(n-1)=3^(-n)