f: 5(x-y)=2(x+y)
=>\(\dfrac{x-y}{2}=\dfrac{x+y}{5}=k\)
=>\(x-y=2k;x+y=5k\)
\(\dfrac{20}{x+y}+\dfrac{20}{x-y}=7\)
=>\(\dfrac{20}{2k}+\dfrac{20}{5k}=7\)
=>\(\dfrac{10}{k}+\dfrac{4}{k}=7\)
=>\(\dfrac{14}{k}=7\)
=>k=2
=>x-y=4 và x+y=10
=>\(\left\{{}\begin{matrix}x-y+x+y=4+10\\x-y=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x=14\\x-y=4\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}x=7\\y=x-4=7-4=3\end{matrix}\right.\)
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