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,求｛an｝通项公式.

学习时间:2026-06-05 02:52:47 阅读：4339

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,求｛an｝通项公式.需步

甜甜的蜜粉

2026-06-05 02:52:47

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3(a1+a3)/2+5(a1+a5)/2=503(2a1+2d)/2+5(2a1+4d)/2=508a1+13d=50a1a13=a4a4a1(a1+12d)=(a1+3d)^212a1d=9d^2a1=3/2d25d=50d=2a1=3an=3+2(n-1)=2n+1