已知递增的等差数列an a1=3且a2-1,a3+1,a8-1成等比求an,Sn令bn=1/Sn...
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发布时间:2024-10-24 12:07
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热心网友
时间:2024-10-24 15:26
由题意得(a3+1)^2=(a2-1)*(a8-1),所以(2d+4)^2=(d+2)*(7d+2),所以d=2
所以an=2n+1,所以sn=n(n+2),所以bn=1/n(n+2),所以bn=1/2n-1/2(n+2),所以Tn=右边相加=1/2+1/4-1/2(n+1)-1/2(n+2)<3/4
热心网友
时间:2024-10-24 15:22
(a3+1)^2=(a2-1)*(a8-1),所以(2d+4)^2=(d+2)*(7d+2),所以d=2
,an=2n+1,所以sn=n(n+2),所以bn=1/n(n+2),所以bn=1/2n-1/2(n+2),Tn=右边相加=1/2+1/4-1/2(n+1)-1/2(n+2)<3/4