Hệ \(\left\{ \begin{array}{l}4x + 5y = 15\\6x - 4y = 11\end{array} \right.\) có nghiệm (m; n).Tính 2m - n

* Hướng dẫn giải

 \(\begin{array}{l}\,\,\left\{ \begin{array}{l}4x + 5y = 15\\6x - 4y = 11\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}12x + 15y = 45\\12x - 8y = 22\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}23y = 23\\4x + 5y = 15\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}y = 1\\4x + 5 = 15\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}y = 1\\4y = 10\end{array} \right.\\ \Leftrightarrow \left\{ \begin{array}{l}x = \dfrac{5}{2}\\y = 1\end{array} \right.\end{array}\)

⇒ 2a - b = 4