a) \(\left\{{}\begin{matrix}x+y=3\\-2x+y=-3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x+y=3\\3x=6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x+y=3\\x=2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}2+y=3\\x=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=1\\x=2\end{matrix}\right.\)
Vậy HPT 1 nghiệm (1;2)
b) \(\left\{{}\begin{matrix}2x+6y=11\\4x-9y=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}4x+12y=22\\4x-9y=1\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}4x-9y=1\\21y=21\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}4x-9.1=1\\y=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2,5\\y=1\end{matrix}\right.\)
Vậy HPT 1 nghiệm (2,5 ; 1)
c) \(\left\{{}\begin{matrix}x+1+y=4\\x+1-2y=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x+y=3\\x=2y\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2y+y=3\\x=2y\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=1\\x=2y\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=1\\x=2\end{matrix}\right.\)
Vậy HPT 1 nghiệm (2 ; 1)
