数列1/3.1/15.1/35.1/63······的前n项和sn
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![数列1/3.1/15.1/35.1/63······的前n项和sn](/uploads/image/z/61729-25-9.jpg?t=%E6%95%B0%E5%88%971%2F3.1%2F15.1%2F35.1%2F63%C2%B7%C2%B7%C2%B7%C2%B7%C2%B7%C2%B7%E7%9A%84%E5%89%8Dn%E9%A1%B9%E5%92%8Csn)
数列1/3.1/15.1/35.1/63······的前n项和sn
数列1/3.1/15.1/35.1/63······的前n项和sn
数列1/3.1/15.1/35.1/63······的前n项和sn
1/3+1/15+1/35+1/63+.+1/(2n-1)(2n+1)
=1/2*[1-1/3+1/3-1/5+1/5-1/7+.+1/(2n-1)-1/(2n+1)]
=1/2*[1-1/(2n+1)]
=n/(2n+1)