Đáp án:
Giải thích các bước giải:
\(\begin{array}{l}
a.\left\{ \begin{array}{l}
\frac{x}{y} = \frac{4}{9}\\
3x - 2y = - 12
\end{array} \right.\\
\to \left\{ \begin{array}{l}
9x - 4x = 0\\
6x - 4x = - 24
\end{array} \right.\\
\to \left\{ \begin{array}{l}
3x = 24\\
9x - 4x = 0
\end{array} \right. \to \left\{ \begin{array}{l}
x = 8\\
y = 18
\end{array} \right.\\
b.\left\{ \begin{array}{l}
\frac{y}{4} = \frac{{ - x}}{3}\\
x - y = 7
\end{array} \right. \to \left\{ \begin{array}{l}
4x + 3y = 0\\
x = 7 + y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = - 4\\
x = 3
\end{array} \right.\\
c.\left\{ \begin{array}{l}
\frac{1}{3}x = \frac{1}{7}y\\
x - y = - 20
\end{array} \right. \to \left\{ \begin{array}{l}
7x - 3y = 0\\
x = - 20 + y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 35\\
x = 15
\end{array} \right.\\
d.\left\{ \begin{array}{l}
\frac{x}{y} = \frac{3}{5}\\
x - y = 10
\end{array} \right. \to \left\{ \begin{array}{l}
5x - 3y = 0\\
x = 10 + y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = - 25\\
x = - 15
\end{array} \right.
\end{array}\)
