$$ \begin{array}{ll} &\begin{cases} x+y=3&\cdots(1)\\ x-y=1&\cdots(2)\\ \end{cases}\\ &\begin{cases} 1x+1y=3&\cdots(1)\\ 1x+(-1)y=1&\cdots(2)\\ \end{cases}\\ (2)+(-1)\times(1)\to(2)'&\\ &\begin{cases} 1x+1y=3&\cdots(1)\\ 0x+(-2)y=-2&\cdots(2)'\\ \end{cases}\\ (2)'\div(-2)\to(2)''&\\ &\begin{cases} 1x+1y=3&\cdots(1)\\ 0x+1y=1&\cdots(2)''\\ \end{cases}\\ (1)+(-1)\times(2)''\to(1)'&\\ \end{array} $$