Coefficiente binomiale

Verificare l’uguaglianza $\displaystyle \binom{k}{2} + \binom{n-k}{2} +k(n-k)- \binom{n}{n-2}=0$ SOLUZIONE $\displaystyle \frac{k!}{2!\cdot (k-2)!}+\frac{(n-k)!}{2!\cdot (n-k-2)!}+kn -k^2-\frac{n!}{(n-2)!\cdot 2!}=$ $=\displaystyle \frac{k(k-1)}{2}+\frac{(n-k)(n-k-1)}{2}+kn -k^2-\frac{n(n-1)}{ 2}=$ $=\displaystyle…