(1) n=1
a1=S1=1-9=-8
(2) n≥2
an=Sn-S(n-1)
=n²-9n-[(n-1)²-9(n-1)]
=2n-10
n=1也满足上式
an=2n-10
所以 -5
已知数列﹛an﹜前n项和,Sn=n²-9n 第k项满足-5＜ak＜8、求k
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