a1=a=9/8 an=(9/8)*q^(n-1)=1/3 q^(n-1)=8/27q^n=(8/27)q Sn=a(1-q^n)/(1-q)=65/24 (1-q^n)/(1-q)=65/27[1-(8/27)q]/(1-q)=65/2727-8q=65-65q q=2/3q^(n-1)=8/27n-1=log(2/3) 8/27 n=3+1=4