Bài 3:
a: \(\left\{{}\begin{matrix}x+y=2\\2x-y=1\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}x+y+2x-y=2+1\\x+y=2\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x=3\\x+y=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=2-x=2-1=1\end{matrix}\right.\)
b: \(\left\{{}\begin{matrix}2x+y=3\\x-y=6\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}2x+y+x-y=3+6\\x-y=6\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x=9\\y=x-6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=3-6=-3\end{matrix}\right.\)
c: \(\left\{{}\begin{matrix}3x-y=7\\x+y=5\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x-y+x+y=7+5\\x+y=5\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}4x=12\\x+y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=3\\y=5-x=5-3=2\end{matrix}\right.\)
Bài 2:
a: \(\left\{{}\begin{matrix}3x+y=3\\2x-y=7\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x+y+2x-y=3+7\\2x-y=7\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}5x=10\\y=2x-7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2\\y=2\cdot2-7=-3\end{matrix}\right.\)
b: \(\left\{{}\begin{matrix}x+2y=5\\3x+4y=5\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}2x+4y=10\\3x+4y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x+4y-3x-4y=10-5\\x+2y=5\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}-x=5\\2y=5-x\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-5\\2y=5-\left(-5\right)=10\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}x=-5\\y=5\end{matrix}\right.\)
c: \(\left\{{}\begin{matrix}2x-y=1\\x+y=2\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}2x-y+x+y=1+2\\x+y=2\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x=3\\y=2-x\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=2-1=1\end{matrix}\right.\)
