\(\left\{{}\begin{matrix}x+y=3\\x+2y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=3-y\\3-y+2y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=3-y\\3+y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=3-2\\y=2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=1\\y=2\end{matrix}\right.\)
\(\left\{{}\begin{matrix}x+y=3\\x+2y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=3-y\\3-y+2y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=2\end{matrix}\right.\)
Vậy hpt có nghiệm (x;y) = (1;2)