\documentclass[10pt,a4paper]{extbook} 
\usepackage{multirow} 
\usepackage{multicol} 
\usepackage{lipsum}
\usepackage[total={18cm,24.5cm},centering,includehead=true]{geometry} 
\setlength{\columnwidth}{85mm} 
\setlength{\columnsep}{10mm}
\renewcommand{\baselinestretch{1.3}}
\makeindex
\headsep=.7cm 
\usepackage[total={12cm,20cm},centering,includehead=true]{geometry}
\input{commands}
\begin{document}
\abovedisplayshortskip=5pt plus 2pt minus 3pt
\belowdisplayshortskip=5pt plus 2pt minus 3pt
\pagenumbering{harfi}
\begin{multicols}{2}
\tableofcontents
\baselineskip=.7cm
\pagenumbering{arabic}
\include{chapter14}
\lipsum[1-10]
\end{multicols}
\include{chapter15}
\lipsum[1-10]
\begin{multicols}{2}
\include{Appendix}
\lipsum[1-10]
\include{Refrences}
\end{multicols}
\end{document}