Latex error: the font size command \normalsize is not defined problem solved

Error description

LaTeX Error: The font size command \normalsize is not defined

resolvent

stay .tex file Be sure to add at the beginning \documentclass{article} sentence , Otherwise, an error will be reported “LaTeX Error: The font size command \normalsize is not defined”.
Such as a correctly functioning LaTeX The sample code is as follows ：

\documentclass{article}
\begin{document}
	$\sum_{i=1}^{n}a_i$
\end{document}

reference

LaTeX Error: The font size command \normalsize is not defined_hnjzsyjyj The column -CSDN Blog