a1q^5-a1q3=216 可化成a1q3(q2-1)=216 (1) a1q^2-a1=8 可化为a1(q2-1) (2) (1)式除以(2)式得q^3=27,q=3把其代入(1)式得a1=1,Sn=a1(1-q^n)/(1-q)=1(1-3^n)/(1-3)=13解得n=3
