$x^2+y^2=\sqrt{x^3}.\sqrt{x}+\sqrt{y^3}.\sqrt{y}\leq \sqrt{x^3+y^3}+\sqrt{x+y}\leq \sqrt{x^2+y^2}+\sqrt{x+y}\leq \frac{1}{2}(x^2+y^2+x+y)$
suy ra $x^2+y^2\leq x+y$
$x^3+y^4-x^2-y^3=x^2(x-1)+y^3(y-1)\leq 0$
suy ra $x\leq 1,y\leq 1$ suy ra $x+y\leq 2$