Giải câu 1 trang 22 sách toán VNEN lớp 9 tập 2.
a) $\left\{\begin{matrix}x - y = 4\\ 3x + 4y = 19\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = y + 4\\ 3(y + 4) + 4y = 19\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = y + 4\\ 7y = 7\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = y + 4\\ y = 1\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = 5\\ y = 1\end{matrix}\right.$
b) $\left\{\begin{matrix}x - \sqrt{3}y = \sqrt{3}\\ \sqrt{3}x + y = 7\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = \sqrt{3}y + \sqrt{3}\\ \sqrt{3}(\sqrt{3}y + \sqrt{3}) + y = 7\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = \sqrt{3}y + \sqrt{3}\\ 4y = 4\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = \sqrt{3}y + \sqrt{3}\\ y = 1\end{matrix}\right.$
$\Leftrightarrow \left\{\begin{matrix}x = 2\sqrt{3}\\ y = 1\end{matrix}\right.$