\documentclass{article}
\usepackage[a4paper,landscape,margin=15mm]{geometry}
\usepackage{amsmath,graphicx}
\newcommand{\exprline}[1]{\par\noindent\begingroup\setbox0=\hbox{$\displaystyle #1$}\ifdim\wd0>\linewidth\resizebox{\linewidth}{!}{\box0}\else\box0\fi\endgroup\par\medskip}
\begin{document}
\noindent\texttt{EXP\_6\_072}\\
Version 5.3.1\\
\exprline{a_{0} = - \frac{35.45092321820688}{\left(0.07250052755844655 + \frac{8.951197831291678}{0.17101141999820696 mchi_{1}}\right) \left(mchi_{1} - 0.4793761720213704\right)}}
\exprline{a_{1} = \frac{1}{\left(\frac{0.2977333128821026}{mchipm_{1} + mchipm_{1}} + \frac{mchi_{1} - mchipm_{1}}{mchi_{1}}\right)^{2}}}
\exprline{a_{2} = \frac{1}{\left(0.07260831044039938 + \frac{8.951199122075424}{0.1709438368060553 mchi_{1}}\right)^{2}}}
\exprline{a_{3} = - \frac{0.2675904440173584}{mchipm_{1}} + \frac{a_{2} mchipm_{1}}{\left(-0.6304848019479864\right) 0.066146833040938411 a_{1}}}
\exprline{a_{4} = - \frac{0.2675904440173584}{a_{3} \left(mchi_{1} - 0.1709438368060553\right)} + \frac{0.6955345839831512}{a_{0}}}
\exprline{a_{5} = a_{4}^{2} - 1.5895574426136225 - - \frac{8.673023774159901}{12261068026780.55 \frac{1}{\left(mchi_{1} - mchipm_{1}\right)^{4}}}}
\exprline{s = a_{5} \cdot 3.6476351559826954 - 0.6181144824152078}
\end{document}