1a)
Đặt \(\frac{x}{4}=\frac{y}{7}=k\Leftrightarrow\left\{{}\begin{matrix}x=4k\\y=7k\end{matrix}\right.\)
Mà \(xy=12\Leftrightarrow28k^2=12\Leftrightarrow k^2=\frac{3}{7}\)
\(\Leftrightarrow\left[{}\begin{matrix}k=\frac{\sqrt{21}}{7}\\k=\frac{-\sqrt{21}}{7}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}\frac{x}{4}=\frac{y}{7}=\frac{\sqrt{21}}{7}\\\frac{x}{4}=\frac{y}{7}=\frac{-\sqrt{21}}{7}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x=\frac{4\sqrt{21}}{7}\\y=\sqrt{21}\end{matrix}\right.\\\left\{{}\begin{matrix}x=\frac{-4\sqrt{21}}{7}\\y=-\sqrt{21}\end{matrix}\right.\end{matrix}\right.\)
Vậy....
