Đáp án:
12) \(\left\{ \begin{array}{l}
y = \dfrac{{155}}{4}\\
x = \dfrac{{93}}{4}\\
z = \dfrac{{31}}{2}
\end{array} \right.\)
Giải thích các bước giải:
\(\begin{array}{l}
3)\left\{ \begin{array}{l}
x = \dfrac{7}{3}y\\
2.\dfrac{7}{3}y + 4y = 68
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{7}{3}y\\
\dfrac{{26}}{3}y = 68
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{7}{3}y\\
y = \dfrac{{102}}{{13}}
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{{238}}{{13}}\\
y = \dfrac{{102}}{{13}}
\end{array} \right.\\
6)\left\{ \begin{array}{l}
x = \dfrac{2}{3}y\\
\dfrac{4}{9}{y^2} + {y^2} = 52
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{2}{3}y\\
\dfrac{{13}}{9}{y^2} = 52
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{2}{3}y\\
{y^2} = 36
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{2}{3}y\\
\left[ \begin{array}{l}
y = 6\\
y = - 6
\end{array} \right.
\end{array} \right.\\
\to \left[ \begin{array}{l}
x = 4\\
x = - 4
\end{array} \right.\\
9)\left\{ \begin{array}{l}
x = \dfrac{2}{3}y\\
z = - \dfrac{4}{3}y\\
3.\dfrac{2}{3}y - 2y = 28
\end{array} \right.\\
\to \left\{ \begin{array}{l}
x = \dfrac{2}{3}y\\
z = - \dfrac{4}{3}y\\
0y = 28\left( l \right)
\end{array} \right.
\end{array}\)
⇒ Không tồn tại giá trị x,y,z thỏa mãn đề bài
\(\begin{array}{l}
12)\left\{ \begin{array}{l}
x = \dfrac{3}{5}y\\
z = \dfrac{2}{5}y\\
5.\dfrac{3}{5}y - y + 3.\dfrac{2}{5}y = 124
\end{array} \right.\\
\to \left\{ \begin{array}{l}
\dfrac{{16}}{5}y = 124\\
x = \dfrac{3}{5}y\\
z = \dfrac{2}{5}y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = \dfrac{{155}}{4}\\
x = \dfrac{3}{5}y\\
z = \dfrac{2}{5}y
\end{array} \right.\\
\to \left\{ \begin{array}{l}
y = \dfrac{{155}}{4}\\
x = \dfrac{{93}}{4}\\
z = \dfrac{{31}}{2}
\end{array} \right.
\end{array}\)
