a)\(\left\{{}\begin{matrix}8y-x=4\\2x-21y=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}16y-2x=8\\2x-21y=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-5y=10\\8y-x=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=-2\\x=-20\end{matrix}\right.\)
a: Ta có: \(\left\{{}\begin{matrix}-x+8y=4\\2x-21y=2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}-2x+16y=8\\2x-21y=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-5y=10\\8y-x=4\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=-2\\x=8y-4=-16-4=-20\end{matrix}\right.\)
b: Ta có: \(\left\{{}\begin{matrix}x+y=-2\left(x-1\right)\\7x+3y=x+y+5\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}3x+y=2\\6x+2y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}6x+2y=4\\6x+2y=5\end{matrix}\right.\)(ptvn)
