Giải :
1)$\left\{\begin{matrix} 2+6y=\dfrac{x}{y}-\sqrt{x-2y}\\ \sqrt{x-\sqrt{x-2y}}=x+3y-2 \end{matrix}\right.$
2)$\left\{\begin{matrix} x^4-y^4=240\\ x^3-2y^3=3(x^2-4y^2)-4(x-8y) \end{matrix}\right.$
3)$\left\{\begin{matrix} \dfrac{2}{\sqrt{x}}+\dfrac{4}{y+2x}=1\\ \dfrac{4}{\sqrt{y}}-\dfrac{4}{y+2x}=1 \end{matrix}\right.$
4)$\left\{\begin{matrix} 5x(1+\dfrac{1}{x^2+y^2})=12\\ 5y(1-\dfrac{1}{x^2+y^2})=4 \end{matrix}\right.$