已知等差数列{an}的前n项和为Sn,若S9=18,Sn=240,an-4=30,则n=( ) A.18 B.17 C.16 D.15
已知等差数列{an}的前n项和为Sn,若S9=18,Sn=240,an-4=30,则n=( )
A. 18
B. 17
C. 16
D. 15
数学人气:775 ℃时间:2020-04-08 22:16:57
优质解答
由等差数列的性质可得S
9=
=
=18,
解得a
5=2,故a
5+a
n-4=32,
而S
n=
=
=16n=240,解得n=15,
故选D
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