Đáp án:
7) \(\left\{ \begin{array}{l}
y = 8\\
x = 12\\
z = 6
\end{array} \right.\)
Giải thích các bước giải:
\(\begin{array}{l}
2)\left\{ \begin{array}{l}
x = \dfrac{9}{8}y\\
z = \dfrac{9}{{12}}y = \dfrac{3}{4}y\\
\dfrac{9}{8}y + y + \dfrac{3}{4}y = 69
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{9}{8}y\\
z = \dfrac{3}{4}y\\
\left( {\dfrac{9}{8} + 1 + \dfrac{3}{4}} \right)y = 69
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{9}{8}y\\
z = \dfrac{3}{4}y\\
\dfrac{{23}}{8}y = 69
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 24\\
x = 27\\
z = 18
\end{array} \right.\\
4)\left\{ \begin{array}{l}
x = \dfrac{3}{8}y\\
z = \dfrac{5}{8}y\\
4.\dfrac{3}{8}y + 3y - 2.\dfrac{5}{8}y = 78
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{3}{8}y\\
z = \dfrac{5}{8}y\\
\dfrac{{13}}{4}y = 78
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 24\\
x = 9\\
z = 15
\end{array} \right.\\
6)\left\{ \begin{array}{l}
x = \dfrac{4}{5}y\\
z = \dfrac{2}{5}y\\
\dfrac{4}{5}y + y - \dfrac{2}{5}y = 21
\end{array} \right.\\
\to \left\{ \begin{array}{l}
\dfrac{7}{5}y = 21\\
x = \dfrac{4}{5}y\\
z = \dfrac{2}{5}y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 15\\
x = 12\\
z = 6
\end{array} \right.\\
7)\left\{ \begin{array}{l}
x = 4 + y\\
4\left( {4 + y} \right) = 6y\\
z = \dfrac{6}{8}y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
2y = 16\\
x = 4 + y\\
z = \dfrac{6}{8}y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = 8\\
x = 12\\
z = 6
\end{array} \right.
\end{array}\)
