设an为等差数列,Sn为数列的前n项和,已知S7=7,S15=75,Tn为数列{Sn/n}的前n项和,求Tn
人气：158 ℃ 时间：2019-09-05 09:27:55
解答
an=a1+(n-1)d sn=na1+nd(n-1)/2
根据这两个式子可以算出 a1=-2 d=1
则sn=-2n+n(n-1)/2
{sn/n}=-2+(n-1)/2
对照an公式可知 A1=-2 D=1/2
Tn=-2n+n(n-1)/4
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