已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=A.260 B.220 C.130 D.110

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已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=A.260 B.220 C.130 D.110
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已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=A.260 B.220 C.130 D.110
已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=
A.260 B.220 C.130 D.110

已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=A.260 B.220 C.130 D.110
a1+a5=1/2
2a3=1/2
a3=1/4
a9=20
a3+a9=2a6=81/4
a6=81/8
S11=11(a1+a11)/2
=22a6/2
=11a6
=891/8
楼主题目没错吧

a1+a5=1/2
2a3=1/2
a3=1/4
a9=20
a3+a9=2a6=81/4
a6=81/8
S11=11(a1+a11)/2
=22a6/2
=11a6
=891/8
楼主题目没错吧学校发的试卷,我也觉得可能错了,怎么算都算不对你看看你题目有没有打错先打错倒没有,难...

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a1+a5=1/2
2a3=1/2
a3=1/4
a9=20
a3+a9=2a6=81/4
a6=81/8
S11=11(a1+a11)/2
=22a6/2
=11a6
=891/8
楼主题目没错吧

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∵a5=a1+4d,2a1+4d=1/2 ①;a9=a1+8d=20 ②
联立①②形成方程组,解之得:a1=-19/3,d=79/24
s11=na1+nd(n-1)/2=? (将a1,d,n=11代入即得)