已知两个等差数列{an}和{bn}的前n项和分别是An和Bn,且AnBn=2n+1n+3,则a9b9等于(  ) A.2 B.74 C.1912 D.1321

问题描述:

已知两个等差数列{an}和{bn}的前n项和分别是An和Bn,且

An
Bn
2n+1
n+3
,则
a9
b9
等于(  )
A. 2
B.
7
4

C.
19
12

D.
13
21

由等差数列的性质可知,

a9
b9
=
2a9
2b9
=
a1+a17
b1+b17
=
A17
B17
=
2×17+1
17+3
=
7
4

故选B