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数学
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an=(-1)~n(n次方)*n,求sn
人气：389 ℃ 时间：2020-06-16 07:08:57
解答
A1=-1,A2=2,A3=-3,A4=4...所以当n为偶数Sn=n/2.所以当n为奇数Sn=-(n+1)/2
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