Đáp án:
5) \(\left\{ \begin{array}{l}
y = 1\\
x = 2
\end{array} \right.\)
Giải thích các bước giải:
\(\begin{array}{l}
1)\left\{ \begin{array}{l}
x + y = 1\\
2x - 3y = 12
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = 1 - y\\
2\left( {1 - y} \right) - 3y = 12
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = 1 - y\\
- 5y = 10
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = 3\\
y = - 2
\end{array} \right.\\
2)\left\{ \begin{array}{l}
x - y = - 1\\
2x + y = 5
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = y - 1\\
2\left( {y - 1} \right) + y = 5
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = y - 1\\
3y = 7
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = \dfrac{7}{3}\\
x = \dfrac{4}{3}
\end{array} \right.\\
3)\left\{ \begin{array}{l}
x + y = 5\\
3x - y = 3
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 3x - 3\\
x + 3x - 3 = 5
\end{array} \right.\\
\to \left\{ \begin{array}{l}
4x = 8\\
y = 3x - 3
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = 2\\
y = 3
\end{array} \right.\\
4)\left\{ \begin{array}{l}
x + y = - 1\\
2x + y = 0
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = - 1 - x\\
2x - 1 - x = 0
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = 1\\
y = - 2
\end{array} \right.\\
5)\left\{ \begin{array}{l}
x - y = 1\\
3x - 4y = 2
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = y + 1\\
3\left( {y + 1} \right) - 4y = 2
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = y + 1\\
- y = - 1
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 1\\
x = 2
\end{array} \right.
\end{array}\)
