x - y = xy = x : y
x - y = xy $\Rightarrow$ x = xy + y = ( x + 1) y
- Nếu x = -1
$\Rightarrow$ - 1 - y = - y $\Rightarrow$ -1 = 0 ( vô lý) (loại )
- Nếu x $\ne$ -1 $\Rightarrow$ y = $\dfrac{x}{x+1}$
x.y = x:y
$\Rightarrow$ x . $\dfrac{x}{x+1}$ = x : $\dfrac{x}{x+1}$
$\Rightarrow$ $\dfrac{x^{2}}{x+1}$ = $\dfrac{x+1}{1}$ ( do x $\ne$ 0)
$\Rightarrow$ $x^{2}$ = $(x+1)^{2}$ = $x^{2}$ +2x+1
$\Rightarrow$ 0 = 2x +1
$\Rightarrow$ x = $\dfrac{-1}{2}$ = - 0,5
$\Rightarrow$ y = $\dfrac{- 0,5}{- 0,5 + 1}$ = -1