Giải:
a) \(xy=7\)
\(\Rightarrow\left(x;y\right)=\left\{\left(1;7\right);\left(-1;-7\right)\right\}\)
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\(\left|x+25\right|+\left|y-5\right|=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}\left|x+25\right|=0\\\left|y-5\right|=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+25=0\\y-5=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=-25\\y=5\end{matrix}\right.\)
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c) \(\left|x-40\right|+\left|x-y+10\right|=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}\left|x-40\right|=0\\\left|x-y+10\right|=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x-40=0\\x-y+10=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=40\\40-y+10=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=40\\50-y=0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=40\\y=50\end{matrix}\right.\)
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