\(\left\{{}\begin{matrix}3x+y=2m+9\\x+y=5\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}3x+y-x-y=2m+9-5\\x+y=5\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}2x=2m+4\\x+y=5\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=m+2\\y=5-x=5-m-2=-m+3\end{matrix}\right.\)
\(A=xy+x-1\)
\(=\left(m+2\right)\left(-m+3\right)+m+2-1\)
\(=-m^2+3m-2m+6+m+1\)
\(=-m^2+2m+7\)
\(=-m^2+2m-1+8\)
\(=-\left(m-1\right)^2+8< =8\)
Dấu '=' xảy ra khi m-1=0
=>m=1
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