∑k^2=n(n+1)(2n+1)/6이고, ∑k=n(n+1)/2이니까 ∑(k^2+k)=n(n+1)(2n+1)/6+n(n+1)/2=n(n+1)((2n+1)/6+1/2))=n(n+1)(n+2)/3이 됩니다.
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