S6=a1(1-q^6)/(1-q)
S3=a1(1-q³)/(1-q)
S6/S3=(1-q^6)/(1-q³)=1+q³=28
q=3
所以13/9=a1(1-3³)/(1-3)
a1=1/9
an=3^(3-n)