\documentclass[12pt]{article} \pagestyle{empty} \begin{document} \begin{displaymath} {{\rho_i}\over{\rho_j}} = { { { \exp[-\beta U({\bf r}_i)] } \over { \int_{\bf r} { \exp[-\beta U({\bf r}) ] } d{\bf r} } } \over { { \exp[-\beta U({\bf r}_j)] } \over { \int_{\bf r} { \exp[-\beta U({\bf r}) ] } d{\bf r} } } } = { {\exp[-\beta U({\bf r}_i)]} \over {\exp[-\beta U({\bf r}_j)]} } \end{displaymath} \end{document}