若等比数列{an}的前n项和为Sn,a2=6,S3=21,则公比q= _ .

问题描述:

若等比数列{an}的前n项和为Sn,a2=6,S3=21,则公比q= ___ .

∵等比数列{an}的前n项和为Sn,a2=6,S3=21,

a1q=6
a1(1-q2)
1-q
=21
,整理得
a1=15
q=
2
5
,解得q=
2
5

故答案为:
2
5