设数列{an}的前n项和为sn=n^2-8n.若Tn为数列{an的绝对值}的前n项的和,求Tn
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设数列{an}的前n项和为sn=n^2-8n.若Tn为数列{an的绝对值}的前n项的和,求Tn
设数列{an}的前n项和为sn=n^2-8n.若Tn为数列{an的绝对值}的前n项的和,求Tn
设数列{an}的前n项和为sn=n^2-8n.若Tn为数列{an的绝对值}的前n项的和,求Tn
Sn - Sn-1 = n^2-8n-[(n-1)^2-8(n-1)]=2n-9=an an为公差为2的等差数列
an
Sn - Sn-1 = n^2-8n-[(n-1)^2-8(n-1)]=2n-9=an an为a1=-7公差为2的等差数列
an<0时n取整数,为4.
当n<5时,an=9-2n,即a1=7公差为-2的等差数列,Tn=a1n+((n-1>n)/2*2=8n-n^2
当n大于或等于5时,Tn=16+a5+a6+a7+a8+……+an=n^2-8n+32
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