Question

{an}为等比数列,若a8/a4=2,S4=4,则a8=?

Answer 1

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