\(\Leftrightarrow7x^2+7y^2=10x^2-20y^2\)
\(\Leftrightarrow27y^2=3x^2\)
\(\Leftrightarrow9y^2=x^2\)
\(\Leftrightarrow81y^4=x^4\)
Vì \(x^4y^4=81\Rightarrow81y^4.y^4=81\Leftrightarrow y^8=1\Leftrightarrow\orbr{\begin{cases}y=1\\y=-1\end{cases}}\)
\(y=\pm1\Rightarrow x^2=9y^2=9\Rightarrow x=\pm3\)
Vậy (x;y)=(\(\pm3;\pm1\))