error sample code

[xiaohongliang2020]

at: https://github.com/xinychen/latex-cookbook/blob/main/chapter-4/section1.ipynb 【例4】

\documentclass[12pt]{article}
\begin{document}

Equation~\eqref{eq1} shows a simple formula.

\begin{equation}\label{eq1}
x+y=2
\end{equation}

\end{document}

error: Undefined control sequence.
It seems shoud use package amsmath to make \eqref work,
so correct code is

\documentclass[12pt]{article}
\usepackage{amsmath}
\begin{document}

Equation~\eqref{eq1} shows a simple formula.

\begin{equation}\label{eq1}
x+y=2
\end{equation}

\end{document}