Đáp án:
` x - y = xy -1`
` => x - y - xy = -1`
` => ( x - xy ) - y =-1`
` => x(1-y) - y = -1`
` => x(1-y) + (1-y) = 0`
` => (x+1)(1-y) = 0`
` => ` \(\left[ \begin{array}{l}x+1=0\\1-y=0\end{array} \right.\)
`=>` \(\left[ \begin{array}{l}x=-1\\y=1\end{array} \right.\)
Vậy ` x = -1` hoặc ` y = 1`
