$\left \{ {(m+1)x +y=m} \atop {x + (m-1)y=2} \right.$
=>x= 2-(m-1)y
=> (m-1)(2-(m-1)y)+y=m
<=> 2m-2-(m-1)^2y +y=m
<=> (1-(m-1)^2)y=2-m
<=> (2m-m^2)y=2-m
<=> y=$\frac{2-m}{2m-m$^{2}$ }$ = $\frac{2-m}{m(2-m)}$ =$\frac{1}{m}$
=> x= 2-(m-1)$\frac{1}{m}$=2-11×$\frac{1}{m}$ = 1+$\frac{1}{m}$ = m+$\frac{1}{m}$
=>x-y= $\frac{m+1}{m}$- $\frac{1}{m}$ =$\frac{m}{m}$ =1 khong phu thuoc vao m
