\(\left\{{}\begin{matrix}mx+\left(m+4\right)y=2\\m\left(x+y\right)=1-y\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}mx+\left(m+4\right)y=2\\mx+\left(m+1\right)y=1\end{matrix}\right.\)
\(D=m\left(m+1\right)-m\left(m+4\right)=-3m\)
\(D_x=2\left(m+1\right)-\left(m+4\right)=m-2\)
\(D_y=m-2m=-m\)
Hệ phương trình vô nghiệm khi \(\left\{{}\begin{matrix}D=0\\\left[{}\begin{matrix}D_x\ne0\\D_y\ne0\end{matrix}\right.\end{matrix}\right.\)\(\Leftrightarrow\left\{{}\begin{matrix}-3m=0\\\left[{}\begin{matrix}m-2\ne0\\-m\ne0\end{matrix}\right.\end{matrix}\right.\)\(\Leftrightarrow m=0\)