\documentclass[a4paper,12pt]{article}
\setbox0=\hbox{$1+2=3\mathbf{x+y}$}
\usepackage{mathptmx}
\begin{document}
\section{\textsf{mathptmx} math output}
\(1+2=3\mathbf{x+y}\)

\section{\LaTeX{} Output}
\usebox0
\end{document}