数列{an}中,an>0,且{anan+1}是公比为q(q>0)的等比数列,满足anan+1+an+1an+2>an+2an+3(n∈N),则公比q的取值范围是( ) A.0<q<1+22 B.0<q<1+52 C.0<q<−1+22 D
问题描述:
数列{an}中,an>0,且{anan+1}是公比为q(q>0)的等比数列,满足anan+1+an+1an+2>an+2an+3(n∈N),则公比q的取值范围是( )
A. 0<q<
1+
2
2
B. 0<q<
1+
5
2
C. 0<q<
−1+
2
2
D. 0<q<
−1+
5
2
答
法1:∵{anan+1}是公比为q(q>0)的等比数列,
∴设anan+1=(a1a2)qn−1,
不等式可化为(a1a2)qn−1+(a1a2)qn>(a1a2)qn+1,
∵an>0,q>0,
∴q2-q-1<0,
解得:0<q<
;1+
5
2
法2:令n=1,不等式变为a1a2+a2a3>a3a4,
∴a1a2+a1a2⋅q>a1a2q2,
∵a1a2>0,∴1+q>q2,
解得:0<q<
,1+
5
2
故选B