\(a,\left\{{}\begin{matrix}x+2y=5\\3x+4y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2y\\3\left(5-2y\right)+4y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2y\\15-6y+4y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2y\\15-2y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=5-2.5\\y=5\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x=-5\\y=5\end{matrix}\right.\)
\(b,\left\{{}\begin{matrix}3x+y=3\\2x-y=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3x\\2y-\left(3-3x\right)=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3x\\2y-3+3x=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3x\\5x-3=7\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=3-3.2\\x=2\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}y=-3\\x=2\end{matrix}\right.\)
\(\left\{{}\begin{matrix}x+2y=5\\3x+4y=5\end{matrix}\right.\)\(\Leftrightarrow\)\(\left\{{}\begin{matrix}3x+6y=15\\3x+4y=5\end{matrix}\right.\Leftrightarrow\)\(\left\{{}\begin{matrix}2y=10\\x+2y=5\end{matrix}\right.\Leftrightarrow\)\(\left\{{}\begin{matrix}y=5\\x+2\times5=5\end{matrix}\right.\Leftrightarrow\)\(\left\{{}\begin{matrix}y=5\\x+10=5\end{matrix}\right.\Leftrightarrow\)\(\left\{{}\begin{matrix}y=5\\x=-5\end{matrix}\right.\)vậy.....
