an+Sn=4 1 a(n+1)+S(n+1)=2a(n+1)+Sn=4 2 2-1得 2a(n+1)- an=0 a(n+1)=1/2 an an+Sn=4 an≠0 a(n+1)/an =1/2 数列{an}是等比数列
