问题 填空题
等差数列{an},{bn}的前n项和分别为Sn、Tn,若
Sn
Tn
=
2n
3n+1
,则
a11
b11
=______.
答案

∵在等差数列中S2n-1=(2n-1)•an

a11=

S21
21
b11=
T21
21

a11
b11
=
S21
T21

又∵

Sn
Tn
=
2n
3n+1

S21
T21
=
21
32

a11
b11
=
21
32

故答案为:

21
32

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