ĐK $y \geq -1$(1) $\Leftrightarrow (\sqrt{x^{2}+2y^{2}}-x-2y)(\sqrt{x^{2}+2y^{2}} +x+y+1)=0$
+) $\sqrt{x^{2}+2y^{2}}=-x-y-1$
(2) $\Leftrightarrow \sqrt{y+1}=3y+1\Leftrightarrow y=0$
thử lại ta thấy k t/m
+) $\sqrt{x^{2}+2y^{2}}=x+2y$
k/h vs (2) $\Rightarrow (x;y)= (\frac{-1-\sqrt{5}}{4};\frac{-1+\sqrt{5}}{2})$