我的文件:
\documentclass{article}
\begin{document}
As known,
\begin{equation}
\label{trivial}
7\times9=63.
\end{equation}
It follows from the formula (\ref{trivial}) that $63/9=7$.
\end{document}
我的警告:
LaTeX Warning: Reference `trivial' on page 1 undefined on input line 9.
我的 PDF:
... from the formula (??) that ...
我的错误是什么?