\(\left\{{}\begin{matrix}2\left(x+y\right)=5\left(x-y\right)\\\frac{20}{x+y}+\frac{20}{x-y}=7\end{matrix}\right.\left(1\right)\) \(Đkxđ:x\ne\pm y\)
\(\Leftrightarrow\left\{{}\begin{matrix}\frac{5}{x+y}=\frac{2}{x-y}\\\frac{20}{x+y}+\frac{20}{x-y}=7\end{matrix}\right.\left(2\right)\)
Đặt: \(\left\{{}\begin{matrix}a=\frac{1}{x+y}\\b=\frac{1}{x-y}\end{matrix}\right.\) Ta có hệ pt \((2)\) trở thành:
\(\left\{{}\begin{matrix}5a=2b\\20a+20b=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}5a-2b=0\\20a+20b=7\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}20a-8b=0\\20a+20b=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}5a=2b\\28b=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}a=\frac{1}{10}\\b=\frac{1}{4}\end{matrix}\right.\)
Với: \(\left\{{}\begin{matrix}a=\frac{1}{10}\\b=\frac{1}{4}\end{matrix}\right.\) Ta lại có hệ pt sau: \(\left\{{}\begin{matrix}\frac{1}{x+y}=\frac{1}{10}\\\frac{1}{x-y}=\frac{1}{4}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x+y=10\\x-y=4\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}2x=14\\x+y=10\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=7\\y=3\end{matrix}\right.\left(tmđk\right)\)
Vậy ........