\documentclass[a4paper,UTF8,fleqn]{book}
\usepackage{geometry}
\geometry{left=2.0cm, right=2.0cm, top=2.5cm, bottom=2.5cm}

\usepackage{ctex}

\usepackage{physics}

\usepackage[colorlinks, linkcolor=black]{hyperref}
\usepackage{amsmath}
\numberwithin{equation}{section}


\usepackage{multicol}
\columnseprule=1pt

% \usepackage{pgfplots}

\setlength{\parindent}{2em}
\setlength{\parskip}{0.5\baselineskip}

\title{习题}
\date{}
\begin{document}
\allowdisplaybreaks[3]
\maketitle
\tableofcontents
\chapter{The Smooth, Expanding Universe}
\input{contents/chap2/derivation.tex}
\section{Exercise 1.}
\input{contents/chap2/ex_01.tex}
% \section{Exercise 2.}
% \input{contents/chap2/ex_02.tex}
% \section{Exercise 3.}
% \input{contents/chap2/ex_03.tex}
% \section{Exercise 4.}
% \input{contents/chap2/ex_04.tex}
% \section{Exercise 5.}\label{sec5}
% \input{contents/chap2/ex_05.tex}
% \section{Exercise 6.}
% \input{contents/chap2/ex_06.tex}
% \section{Exercise 7.}
% \input{contents/chap2/ex_07.tex}
% \section{Exercise 8.}
% \input{contents/chap2/ex_08.tex}
% \section{Exercise 9.}
% \input{contents/chap2/ex_09.tex}
% \section{Exercise 10.}
% \input{contents/chap2/ex_10.tex}
% \section{Exercise 11.}
% \input{contents/chap2/ex_11.tex}
% \section{Exercise 12.}\label{1:12}
% \input{contents/chap2/ex_12.tex}
% \section{Exercise 13.}
% \input{contents/chap2/ex_13.tex}
% \section{Exercise 14.}
% \input{contents/chap2/ex_14.tex}
% \section{Exercise 15.}
% \input{contents/chap2/ex_15.tex}
% \section{Exercise 16.}
% \input{contents/chap2/ex_16.tex}
% \section{Exercise 17.}
% \input{contents/chap2/ex_17.tex}
% \section{Exercise 18.}
% \input{contents/chap2/ex_18.tex}
\section{Exercise 19.}
\input{contents/chap2/ex_19.tex}

\chapter{Beyond Equilibrium}
\end{document}