\(x+y=0\) và \(x-y=0\) không phải là nghiệm
\(\Leftrightarrow\left\{{}\begin{matrix}\left(x+y\right)\left(x-y\right)^2=3\\\left(x+y\right)\left(x^2+y^2\right)=15\end{matrix}\right.\)
\(\Rightarrow\frac{x^2+y^2}{\left(x-y\right)^2}=5\)
\(\Leftrightarrow x^2+y^2=5x^2-10xy+5y^2\)
\(\Leftrightarrow2x^2-5xy+2y^2=0\)
\(\Leftrightarrow\left(2x-y\right)\left(x-2y\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=2y\\y=2x\end{matrix}\right.\) \(\Rightarrow\left[{}\begin{matrix}\left(2y-y\right)\left(4y^2-y^2\right)=3\\\left(x-2x\right)\left(x^2-4x^2\right)=3\end{matrix}\right.\)
