\(\left\{{}\begin{matrix}4x+5y=14\\5x+4y=13\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{14-5y}{4}\\\dfrac{70-25y}{4}+4y=13\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{14-5y}{4}\\\dfrac{70-9y}{4}=13\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{14-5y}{4}\\y=\dfrac{70-\left(13.4\right)}{9}\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=\dfrac{14-5.2}{4}\\y=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=2\end{matrix}\right.\)
thay x = 1 ; y=2 và 9x + 9y ta đc
\(\Leftrightarrow9+9.2=9+18=27\)
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