a, Thay $m=-1$ vào, ta có: $\left \{ {{x+y=-1} \atop {2x+y=0}} \right.$
⇔ $\left \{ {{x-1} \atop {y=-2}} \right.$
b, Để pt vô nghiệm thì: $-m=2\neq0$
⇔ $m=-2$
c, $\left \{ {{x+y=m} \atop {2x-my=0}} \right.$
⇔ $\left \{ {{2x+2y=2m} \atop {2x-my=0}} \right.$
⇔ $\left \{ {{2y+my=2} \atop {x=m-y}} \right.$
⇔ $\left \{ {{y=\frac{2}{m+2}} \atop {x=m-\frac{2}{m+2}=\frac{m²+2m-2}{m+2}}} \right.$
Vì $x+y=1$
⇒ $\frac{2}{m+2}+\frac{m²+2m-2}{m+2}=1$
⇔ $\frac{m²+2m}{m+2}=1$
⇔ $m=1$
