Result
R2782: Arithmetic expression for sum of initial natural numbers shows that
\begin{equation}
\sum_{n = 0}^N n
= \frac{N (N + 1)}{2}
\end{equation}
while from result
R1831: Real arithmetic expression for binomial coefficient, we have
\begin{equation}
\binom{N + 1}{2}
= \frac{(N + 1) !}{(N + 1 - 2) ! 2 !}
= \frac{(N + 1) !}{(N - 1) ! 2 !}
= \frac{N (N + 1)}{2}
\end{equation}
$\square$
