\documentclass[a4paper,10pt]{article}

\usepackage[nocjkgb]{ucs}
\usepackage[utf8x]{inputenc}
\usepackage[C10,T1]{fontenc}
\usepackage{srcltx}
\newcommand\chinesesimptext[1]{\bgroup\fontencoding{C10}\fontfamily{song}\selectfont\SetUnicodeOption{cjkgb}#1\egroup}
%opening
\title{}
\author{}

\begin{document}
\maketitle

\begin{abstract}
\end{abstract}


\[
q(x,t)=
\begin{cases}(t-k+1)x^2,\quad \ \ 
  t\in\big(k-1,k-\big],\\
  (k-t)x^2, \quad \ \  t\in\big(k-1,k\big],
\end{cases}
\]

\end{document}