(2011•三亚模拟)在等比数列{an}中,a1+an=34,a2•an-1=64,且前n项和Sn=62,则项数n等于(  ) A.4 B.5 C.6 D.7

问题描述:

(2011•三亚模拟)在等比数列{an}中,a1+an=34,a2•an-1=64,且前n项和Sn=62,则项数n等于(  )
A. 4
B. 5
C. 6
D. 7

因为数列{an}为等比数列,则a2•an-1=a1•an=64①,
又a1+an=34②,
联立①②,解得:a1=2,an=32或a1=32,an=2,
当a1=2,an=32时,sn=

a1(1−qn)
1−q
=
(a1anq)
1−q
=
2−32q
1−q
=62,
解得q=2,所以an=2×2n-1=32,此时n=5;
同理可得a1=32,an=2,也有n=5.
则项数n等于5
故选B