>>555
Σ_[n=1,k]Σ_[m=1,n]2m=Σ_[n=1,k] n(n+1)
=(Σ_[k=1,m] k^2)+(Σ_[k=1,m] k)
=(1/6)*k(k+1)(2k+1)+(1/2)*k(k+1)
=(1/6)*k(k+1)(2k+4)
=(1/3)*k(k+1)(k+2)