数列an是首项为3公差为2的等差数列其前n项和为Sn求An=1/S1+1/S2+1/S3+...+1/Sn
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![数列an是首项为3公差为2的等差数列其前n项和为Sn求An=1/S1+1/S2+1/S3+...+1/Sn](/uploads/image/z/2779517-29-7.jpg?t=%E6%95%B0%E5%88%97an%E6%98%AF%E9%A6%96%E9%A1%B9%E4%B8%BA3%E5%85%AC%E5%B7%AE%E4%B8%BA2%E7%9A%84%E7%AD%89%E5%B7%AE%E6%95%B0%E5%88%97%E5%85%B6%E5%89%8Dn%E9%A1%B9%E5%92%8C%E4%B8%BASn%E6%B1%82An%3D1%2FS1%2B1%2FS2%2B1%2FS3%2B...%2B1%2FSn)
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数列an是首项为3公差为2的等差数列其前n项和为Sn求An=1/S1+1/S2+1/S3+...+1/Sn
数列an是首项为3公差为2的等差数列其前n项和为Sn求An=1/S1+1/S2+1/S3+...+1/Sn
数列an是首项为3公差为2的等差数列其前n项和为Sn求An=1/S1+1/S2+1/S3+...+1/Sn
an = a1 + (n-1)d = 2n + 1
Sn = [n(a1 + an)]/2 = n(n+2)
1/Sn = 1/n(n+2) = [1/n - 1/(n+2)]/2
An = S1 + S2+ ...+ Sn = 21/40 - (2n+3)/2(n+2)(n+1)