设等差数列An的前n项和为Sn,Bn=1/Sn,且A3*B3=1/2,S5+S3=21,
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![设等差数列An的前n项和为Sn,Bn=1/Sn,且A3*B3=1/2,S5+S3=21,](/uploads/image/z/14843497-49-7.jpg?t=%E8%AE%BE%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97An%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%2CBn%3D1%2FSn%2C%E4%B8%94A3%2AB3%3D1%2F2%2CS5%2BS3%3D21%2C)
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设等差数列An的前n项和为Sn,Bn=1/Sn,且A3*B3=1/2,S5+S3=21,
设等差数列An的前n项和为Sn,Bn=1/Sn,且A3*B3=1/2,S5+S3=21,
设等差数列An的前n项和为Sn,Bn=1/Sn,且A3*B3=1/2,S5+S3=21,
S5=5A3 ,B3=1/S3 题设变成:A3/S3=1/2,5A3+S3=21 解方程得:A3=3,S3=6 设等差数列差为d S3=A3+A3-d+A3-2d d=1,A1=A3-2d=1 Sn=n(n+1)/2 B2=2/(nn+n)