a)
\(\left\{{}\begin{matrix}3x-y=5\\x+2y=4\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}3x-y=5\\x=4-2y\end{matrix}\right.\\ \Leftrightarrow3\left(4-2y\right)-y=5\\ \Leftrightarrow12-6y-y-5=0\\ \Leftrightarrow7-7y=0\\ \Leftrightarrow7y=7\\ \Leftrightarrow y=1\\ \Leftrightarrow x=4-2.1=4-2=2\)
Vậy HPT có tập nghiệm \(S=\left\{x;y\right\}=\left\{2;1\right\}\)
b)
\(\left\{{}\begin{matrix}4x-y=7\\5x+y=2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=4x-7\\5x+y=2\end{matrix}\right.\\ \Leftrightarrow5x+4x-7=2\\ \Leftrightarrow9x-7-2=0\\ \Leftrightarrow9x-9=0\\ \Leftrightarrow9x=9\\ \Leftrightarrow x=1\\ \Leftrightarrow y=4.1-7=-3\)
Vậy HPT có tập nghiệm \(S=\left\{x;y\right\}=\left\{1;-3\right\}\)
