I have problem with prooving those two identities. Any help would be much appriciated!
Show that:
a)
\begin{Bmatrix}
m+n+1\\ m
\end{Bmatrix}
= \sum_{k=0}^{m} k \begin{Bmatrix}
n+k\\k
\end{Bmatrix}
b)
\sum_{k=0}^{n} \begin{pmatrix}
n\\k
\end{pmatrix}...