已知等差数列｛an｝前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13,成等比数列设{bn/an}是首

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已知等差数列｛an｝前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13,成等比数列设{bn/an}是首项为1,公比为

3a1+3*2/2d+5a1+5*4/2d=50
3a1+3d+5a1+10d=50
8a1+13d=50
a4^2=a1a13
(a1+3d)^2=a1(a1+12d)
a1^2+6a1d+9d^2=a1^2+12a1d
6a1d-9d^2=0
2a1d-3d^2=0
d(2a1-3d)=0
d=0or2a1-3d=0
d/=0
2a1-3d=0
8a1-12d=0
8a1+13d=50
25d=50
d=2
a1=3d/2=3
an=a1+(n-1)d=3+(n-1)2=3+2n-2=2n+1
(n:N*)
再问：公比为3等比求前N项和

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已知等差数列｛an｝前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13,成等比数列 设{bn/an}是首

已知等差数列｛an｝前n项和为Sn,公差d≠0,且S3+S5=50,a1,a4,a13,成等比数列设{bn/an}是首