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解 n(n+2)=n²;+2n ∴sn =(1²;+2²;+3²;+……+n²;)+2(1+2+3+……+n) =n*(n+1)*(2n+1)/6+2×(1+n)n/2 =1/6n(n+1)(2n+1)+(1+n)n =n(n+1)(1/3n+1/6+1) =n(n+1)(1/3n+7/6) 公式1²;+2²;+……+n²;=n*(n+1)*(2n+1)/6
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