\documentclass{article}

\input{../../../preamble}

\newcommand{\lean}[1]{\leanref
  {./Arithmetic.html\#Real.Arithmetic.#1}
  {Real.Arithmetic.#1}}

\begin{document}

\section{\proceeding{Sum of Arithmetic Series}}%
\label{sec:sum-arithmetic-series}

Let $(a_i)_{i \geq 0}$ be an arithmetic sequence with common difference $d$.
Then for some $n \in \mathbb{N}$,
$$\sum_{i=0}^n a_i = \frac{(n + 1)(a_0 + a_n)}{2}.$$

\begin{proof}

  \lean{sum\_recursive\_closed}

\end{proof}

\end{document}
