Nhận thấy \(x=0\Rightarrow y=0\) là 1 cặp nghiệm và ngược lại
Với \(x;y\ne0\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+y+xy\left(x+y\right)=18xy\\x^2+y^2+x^2y^2\left(x^2+y^2\right)=208x^2y^2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+y+\frac{1}{x}+\frac{1}{y}=18\\x^2+y^2+\frac{1}{x^2}+\frac{1}{y^2}=208\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x+\frac{1}{x}+y+\frac{1}{y}=18\\\left(x+\frac{1}{x}\right)^2+\left(y+\frac{1}{y}\right)^2=212\end{matrix}\right.\)
Đặt \(\left\{{}\begin{matrix}x+\frac{1}{x}=a\\y+\frac{1}{y}=b\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}a+b=18\\a^2+b^2=212\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}a+b=18\\\left(a+b\right)^2-2ab=212\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}a+b=18\\ab=56\end{matrix}\right.\)
Theo Viet đảo, a và b là nghiệm của:
\(t^2-18t+56=0\Rightarrow\left[{}\begin{matrix}t=4\\t=14\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x+\frac{1}{x}=4\\y+\frac{1}{y}=14\end{matrix}\right.\\\left\{{}\begin{matrix}x+\frac{1}{x}=14\\y+\frac{1}{y}=4\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x^2-4x+1=0\\y^2-14y+1=0\end{matrix}\right.\\\left\{{}\begin{matrix}x^2-14x+1=0\\y^2-4y+1=0\end{matrix}\right.\end{matrix}\right.\) \(\Leftrightarrow...\)
