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

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