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幼苗
共回答了8个问题采纳率:87.5% 举报
(I)因bn=an+1-an,所以bn+1=an+2-an+1=5/3an+1-2/3an-an+1=2/3(an+1-an)=2/3bn
故{bn}是公比为2/3的等比数列,且b1=a2-a1=2/3
x09(II)由 bn=an+1-an
得an+1-a1=(an+1-an)+(an-an-1)+……+(a2-a1)
=(2/3)n次方+(2/3)n-1次方+……+(2/3)
设Tn={n2n-1次方/3n-1次方}
a1=1,得an=3-2n次方/3n-1次方
Tn-1/3Tn=9-(3+n)2n次方/3n-1次方
Sn=a1+2a2+……+nan=3/2n(n+1)+(3+n)2n次方+1/3n-1次方 -18
1年前
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