Question

设等比数列{an}的前n项和为Sn，若S6/S3=3.则S9/S6

答案是7/3，求解析

Answer 1

解：
S6/S3=(1-q^6)/(1-q^3)=1+q^3=3
q^3=2
S9/S6=(1-q^9)/(1-q^6)
=(1-8)/(1-4)
=7/3