Bài 3.4: Đặt x – y = x.y = x : y = t
$\Rightarrow \left\{ \begin{align} & x=y+t \\ & x=\frac{t}{y} \\ & x=t.y \\ \end{align} \right.$$\Rightarrow \frac{t}{y}=t.y$$\Rightarrow {{y}^{2}}=1$$\Rightarrow y=\pm 1$(tm $y\ne 0$)
Mà $t+y=t.y$$\Rightarrow \left[ \begin{align} & t+1=t.1 \\ & t+\left( -1 \right)=t.\left( -1 \right) \\
\end{align} \right.$$\Rightarrow \left[ \begin{align} & t\in \varnothing \\ & 2t=1 \\
\end{align} \right.$$\Rightarrow t=\frac{1}{2}$$\Rightarrow x=\frac{-1}{2}$
Vậy $\left( x,y \right)=\left( \frac{-1}{2};-1 \right)$