18/ $\left\{\begin{matrix} 2x^3+y(x+1)=4x^2 & \\ 5x^4-4x^6=y^2 & \end{matrix}\right.$
19/ $\left\{\begin{matrix} x^2+2y^2-3xy+x-y=0 & \\ x^2-3y+4y^2-1=0 & \end{matrix}\right.$
20/ $\left\{\begin{matrix} 2x^3+2x^2y-xy=y^2-x-y & \\ 2x^3-xy+x^2=4 & \end{matrix}\right.$
21/ $\left\{\begin{matrix} 2x^2-8xy^2-xy+4y^3=0 & \\ 16x^3+2x-8y^2+5=0 & \end{matrix}\right.$