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
答
∵4a1,2a2,a3成等差数列
∴
= 2a2,4a1+a3
2
∴
=2a1q,即4a1+a1•q2
2
=2 q4+q2
2
∴q=2
∴S4=
=
a1(1−q4) 1−q
=151×(1−24) 1−2
故选C