Stirling numbers - hard proofs

Meekah
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I have problem with prooving those two identities. Any help would be much appriciated!

Show that:

a)
\begin{Bmatrix}<br /> <br /> m+n+1\\ m <br /> <br /> \end{Bmatrix}<br /> <br /> = \sum_{k=0}^{m} k \begin{Bmatrix}<br /> <br /> n+k\\k <br /> <br /> \end{Bmatrix}<br />

b)<br /> \sum_{k=0}^{n} \begin{pmatrix}<br /> <br /> n\\k <br /> <br /> \end{pmatrix}<br /> <br /> \begin{Bmatrix}<br /> <br /> k\\m <br /> <br /> \end{Bmatrix}<br /> <br /> = \begin{Bmatrix}<br /> <br /> n+1\\m+1 <br /> <br /> \end{Bmatrix} <br /> <br />

Where:
\begin{Bmatrix}

k\\m

\end{Bmatrix}
is a Stirling number of the second kind.
 
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what have u tried?
 
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