Đáp án:
d) m=5
Giải thích các bước giải:
\(\begin{array}{l}
a)Thay:m = 7\\
Hpt \to \left\{ \begin{array}{l}
x - y = - 1\\
x + y = 7
\end{array} \right.\\
\to \left\{ \begin{array}{l}
2x = 6\\
y = x + 1
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = 3\\
y = 4
\end{array} \right.\\
b)Thay:x = 0;y = 1\\
Hpt \to \left\{ \begin{array}{l}
1 = 0 + 1\left( {ld} \right)\\
0 + 1 - m = 0
\end{array} \right.\\
\to m = 1\\
c)\left\{ \begin{array}{l}
x - y = - 1\\
x + y = m
\end{array} \right.\\
\to \left\{ \begin{array}{l}
2x = m - 1\\
y = x + 1
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{{m - 1}}{2}\\
y = \dfrac{{m - 1}}{2} + 1 = \dfrac{{m - 1 + 2}}{2}
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{{m - 1}}{2}\\
y = \dfrac{{m + 1}}{2}
\end{array} \right.
\end{array}\)
⇒ Không tồn tại m để hệ vô nghiệm
\(\begin{array}{l}
d)Do:x + y = 5\\
\to \dfrac{{m - 1}}{2} + \dfrac{{m + 1}}{2} = 5\\
\to m - 1 + m + 1 = 10\\
\to 2m = 10\\
\to m = 5
\end{array}\)
