納k=1,n] (k+n)(k-n)
=納k=1,n] (k^2-n^2)
=納k=1,n] k^2 - 納k=1,n] n^2
=(1/6)n(n+1)(2n+1)-n^3
=(1/6)(2n^3+3n^2+n)-n^3
=(1/6)(-4n^3+3n^2+n)
=(1/6)n(-4n^2+3n+1)