Hệ phương trình \(\left\{ \...

B

- Hướng dẫn giải

 $\begin{array}{l}\left\{ \begin{array}{l}2x + 5y = 1\\6x - 15y = 4\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}x = \dfrac{{1 - 5y}}{2}\\6.\dfrac{{1 - 5y}}{2} - 15y = 4\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}x = \dfrac{{1 - 5y}}{2}\\3\left( {1 - 5y} \right) - 15y = 4\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}x = \dfrac{{1 - 5y}}{2}\\3 - 15y - 15y = 4\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}x = \dfrac{{1 - 5y}}{2}\\ - 30y = 1\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}y = \dfrac{{ - 1}}{{30}}\\x = \dfrac{{1 - 5.\dfrac{{ - 1}}{{30}}}}{2} = \dfrac{{1 + \dfrac{1}{6}}}{2} = \dfrac{7}{{12}}\end{array} \right.\end{array}$

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