In de driehoek van Pascal:
\(
\begin{array}{l}
\left( {\begin{array}{*{20}c}
n \\
k \\
\end{array}} \right) + 2 \cdot \left( {\begin{array}{*{20}c}
n \\
{k - 1} \\
\end{array}} \right) + \left( {\begin{array}{*{20}c}
n \\
{k - 2} \\
\end{array}} \right) = \\
\left( {\begin{array}{*{20}c}
n \\
{k - 2} \\
\end{array}} \right) + \left( {\begin{array}{*{20}c}
n \\
{k - 1} \\
\end{array}} \right) + \left( {\begin{array}{*{20}c}
n \\
{k - 1} \\
\end{array}} \right) + \left( {\begin{array}{*{20}c}
n \\
k \\
\end{array}} \right) = \\
\left( {\begin{array}{*{20}c}
{n + 1} \\
{k - 1} \\
\end{array}} \right) + \left( {\begin{array}{*{20}c}
{n + 1} \\
k \\
\end{array}} \right) = \\
\left( {\begin{array}{*{20}c}
{n + 2} \\
k \\
\end{array}} \right) \\
\end{array}
\)
