等比数列{an}中,an∈R,a1+a5=34,a5-a1=30,则a3的值是( ) A.8 B.-6 C.±8 D.16
问题描述:
等比数列{an}中,an∈R,a1+a5=34,a5-a1=30,则a3的值是( )
A. 8
B. -6
C. ±8
D. 16
答
由a1+a5=34,a5-a1=30,联立
解得
a1+a5=34
a5−a1=30
.
a1=2
a5=32
又
=a1a5=64,解得a3=±8.
a
23
设等比数列的公比为q,则a3=a1q2>0,因此a3=-8应舍去.
故a3=8.
故选:A.