| (i−1) i (i+1)={(i−1) i (i+1)・(i+2)−(i−2)・(i−1) i (i+1)}/4 4(i−1) i (i+1)=(i−1) i (i+1) (i+2)−(i−2) (i−1) i (i+1) Σ[i=1,n] 4(i−1) i (i+1)=Σ[i=1,n] (i−1) i (i+1) (i+2)−Σ[i=1,n] (i−2) (i−1) i (i+1) Σ[i=1,n] 4(i−1) i (i+1)=(n−1) n (n+1) (n+2)−0 4Σ[i=1,n] (i^3−i)=(n−1) n (n+1) (n+2) 4Σ[i=1,n] i^3=(n−1) n (n+1) (n+2)+2n(n+1)=n(n+1)(n^2+n) Σ[i=1,n] i^3={n(n+1)/2}^2
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