a2an-1=128=a1ana1+an=66解得a1=64,an=2,Sn=(a1-anq)(1-q)=(64-2q)/(1-q)=126q=1/2,an/a1=1/32=q^(n-1)n=6或a1=2,an=64,Sn=(a1-anq)(1-q)=(2-64q)/(1-q)=126q=2,an/a1=32=q^(n-1)n=6