\documentclass[10pt,dvips]{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage[usenames]{color}
\usepackage{ifxetex}
\usepackage{ifluatex}

% XeLaTeX compiler
\ifxetex

    \usepackage{fontspec}
    \usepackage{unicode-math}

    % Uncomment these lines for alternative fonts
    %\setmainfont{FreeSerif}
    %\setmathfont{FreeSerif}

% LuaLaTeX compiler
\else\ifluatex

    \usepackage{fontspec}
    \usepackage{lualatex-math}

% LaTeX compiler
\else

    % Uncomment this line for sans-serif maths font
    %\everymath{\mathsf{\xdef\mysf{\mathgroup\the\mathgroup\relax}}\mysf}

\fi\fi

% Uncomment this line for custom color
%\definecolor{mycolor}{RGB}{0, 130, 180}



\pagestyle{empty}
\begin{document}
\varepsilon = \frac{RMS-RMS_{mean}}{RMS_{mean}}\times 100
\end{document}