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當n是偶數時,Sn=(-1+3)+(-5+7)+…+[-(2n-3)+(2n-1)]=2+2+…+2(共n2項)=2×n2=n.當n是奇數時,Sn=(-1+3)+(-5+7)+…+[-(2n-5)+(2n-3)]+(-1)×(2n-1)=2+2+…+2(共有n−12項)-(2n-1)=2×(n…
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