$x-y=x^2+xy+y^2\Rightarrow x-xy-y+1=(x^2-2xy+y^2)+1\Rightarrow x(1-y)+(1-y)=(x-y)^2+1\Rightarrow (x+1)(1-y)=(x-y)^2+1\geqslant 1\Rightarrow (x+1)(y-1)\leqslant -1$
+/ Nếu $y=0$ $\Rightarrow $ $x=0$ hoặc $x=1$
+/ Nếu $y$ khác $0$ $\Rightarrow $ $y\geqslant 1\Rightarrow y-1\geqslant 0\Rightarrow (x+1)(y-1)\geqslant 0$(Vô lý)