\(\left\{{}\begin{matrix}2x=5y\\x-y=9\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x-5x=0\\x-y=9\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x-5y=0\\2x-2y=18\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3y=18\\2x-2y=18\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=6\\x=15\end{matrix}\right.\)
b) \(\dfrac{2x}{3}=\dfrac{4y}{7}\Rightarrow\dfrac{x}{1,5}=\dfrac{y}{1,75}\)
Áp dụng tỉ số của dãy số bằng nhau, ta có: \(\dfrac{x+y}{1,5+1,75}=\dfrac{39}{3,25}=12\)
\(\dfrac{2x}{3}=12\Rightarrow x=18\)
\(\dfrac{4y}{7}=12\Rightarrow y=21\)
