Item 1.4.2.c.
Use your answers to parts (a) and (b) to give a combinatorial proof of the identity
\begin{equation*}
{x+y \choose 2} - {x \choose 2} - {y \choose 2} = xy.\text{.}
\end{equation*}
Use your answers to parts (a) and (b) to give a combinatorial proof of the identity