\documentclass{article}
\begin{document}
$36x^2y+54xy^2$
{\boldmath $36x^2y+54xy^2$} % use { and } to localize scope of \boldmath
$36x^2y+54xy^2$
\end{document}
答案2
含bm包装:
\documentclass[margin=3mm]{standalone}
\usepackage{bm}
\begin{document}
I tried $\bm{36x^2y+54xy^2}$
\end{document}