{an}为等比数列,若a8/a4=2,S4=4,则a8=?

题目

{an}为等比数列,若a8/a4=2,S4=4,则a8=?

答案

错了,应该求S8
a8/a4=q^4=2
S4=a1(1-q^4)/(1-q)=4
a1/(1-q)=-4
所以S8=a1(1-q^8)/(1-q)
=-4*(1-2²)
=12