4/ $\left\{\begin{matrix} \sqrt{x-2y+6}=8x^4-8x^2+3 & \\ \frac{5+\sqrt{x-2y+6}}{1+\sqrt{x-2y+6}}=2x^2 & \end{matrix}\right.$
5/ $\left\{\begin{matrix} y+xy^2=-6x^2 & \\ 1+x^3y=19x^3 & \end{matrix}\right.$
6/ $\left\{\begin{matrix} \sqrt{11+x-y}-\sqrt{y-x}=1 & & \\ 7\sqrt{y-x}+6y-26x=3 & & \end{matrix}\right.$