(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
答
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
=(a1−anq) 1−q
=62,2−32q 1−q
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