```tex \documentclass{article} \usepackage{amsmath} \begin{document} \[ Q=\left\{(x,y)\left|\left\{ \begin{array}{l} y\leq x\\ y-1\geq 0\\ x+y-4\leq 0 \end{array} \right\},.\right.\right. \] \end{document} ```  ```tex \documentclass{ctexart} \usepackage{amsmath} \begin{document} 其实想用也不是不能用，只不过这种方式排出来太丑了，不信你看： 已知集合$ Q=\left\{(x,y)\left|\left\{ \begin{array}{l} y\leq x\\ y-1\geq 0\\ x+y-4\leq 0 \end{array} \right\}\right.\right. $，我们现在需要blablabla\ldots \end{document} ```