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
,则2n+1 n+3
等于( )a9 b9
A. 2
B.
7 4
C.
19 12
D.
13 21
答
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