Câu 7: \(\left\{{}\begin{matrix}-2x+y=3\\x-4y=1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-2x+y=3\\2x-8y=2\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}-2x+y+2x-8y=3+2\\x-4y=1\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}-7y=5\\x=4y+1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=-\dfrac{5}{7}\\x=4\cdot\dfrac{-5}{7}+1=-\dfrac{20}{7}+1=-\dfrac{13}{7}\end{matrix}\right.\)
=>Chọn A
Câu 8: \(\left\{{}\begin{matrix}x+y=3\\3x-4y=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3x+3y=9\\3x-4y=4\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x+3y-3x+4y=9-4\\x+y=3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}7y=5\\x=3-y\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}y=\dfrac{5}{7}\\x=3-\dfrac{5}{7}=\dfrac{16}{7}\end{matrix}\right.\)
=>Chọn B
Câu 9:
\(\left\{{}\begin{matrix}x-2y=3\\3x+4y=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x-4y=6\\3x+4y=4\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}5x=10\\x-2y=3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2\\2y=x-3=2-3=-1\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}x=2\\y=-\dfrac{1}{2}\end{matrix}\right.\)
=>Chọn A
Câu 10:
\(\left\{{}\begin{matrix}2x-y=3\\3x-y=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}3x-y-2x+y=4-3\\3x-y=4\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}x=1\\y=3x-4=3\cdot1-4=-1\end{matrix}\right.\)
=>Chọn B
