(2011•顺义区二模)已知等比数列{an}中,各项都是正数,且a1,12a3,2a2成等差数列,则a6+a7a8+a9等于( ) A.1+2 B.1−2 C.3+22 D.3−22
问题描述:
(2011•顺义区二模)已知等比数列{an}中,各项都是正数,且a1,
a3,2a2成等差数列,则1 2
等于( )
a6+a7
a8+a9
A. 1+
2
B. 1−
2
C. 3+2
2
D. 3−2
2
答
设等比数列{an}的公比为q,
∵各项都是正数,且a1,
a3,2a2成等差数列,1 2
∴a3=a1+2a2,即 a1q2=a1+2a1q,解得 q=1+
,或q=1-
2
(舍去).
2
∴
=
a6+a7
a8+a9
=
a6+a6q
a6q2+a6q3
=1+q
q2(1+q)
=3-21 q2
.
2
故选D.