> 数学 >
若两个等差数列{an}和{bn}的前n项和分别是Sn和Tn,已知
Sn
Tn
=
n
2n+1
,则
a7
b7
等于(  )
A.
13
21

B.
21
4

C.
13
27

D.
8
27
人气:232 ℃ 时间:2019-11-13 05:28:20
解答
Sn
Tn
=
n
2n+1

a7
b7
=
2a7
2b7
=
13
2
(a1+a13)
13
2
(b1+b13)
=
S13
T13
=
13
2×13+1
=
13
27

故选:C.
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