数列的前N项和为Sn,An=Sn乘以S(n-1),A1=2/9,求An
人气：351 ℃ 时间：2020-06-03 14:17:07
解答
an=Sn*S(n-1）Sn-S(n-1)=Sn*S(n-1）1/Sn-1/S(n-1)=-1所以{1/Sn}是以1/S1=9/2为首相,公差d=-1的等差数列1/Sn=9/2-1(n-1)=-n+11/2Sn=2/(11-2n)即an=Sn-S(n-1)=2/(11-2n)-2/[11-2(n-1)]=4/(11-2n)(13-2n)=4/(2n-11)(2n-...
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