等比数列{an}的前n项和为Sn,且4a1,2a2,a3成等差数列.若a1=1,则S4=(  ) A.7 B.8 C.15 D.16

问题描述:

等比数列{an}的前n项和为Sn,且4a1,2a2,a3成等差数列.若a1=1,则S4=(  )
A. 7
B. 8
C. 15
D. 16

∵4a1,2a2,a3成等差数列

4a1+a3
2
= 2a2
4a1+a1q2
2
=2a1q
,即
4+q2
2
=2 q

∴q=2
∴S4=
a1(1−q4)
1−q
=
1×(1−24)
1−2
=15
故选C