Differential Equations

$\left(x^2-y^2\right)^2=x^2+y^2$

$\left(x^2-y^2\right)^2=x^2+y^2$

$\left(x^2+y\right)^2=x^2-y^2$

$x^2-y=\left(x+y^2\right)^2$