1. Given $x>0, y>0, x^{2}+y^{2}=1$. Find the minimum value of $\frac{x^{6}+y^{6}}{x y}$.
(Tip: $\frac{x^{6}+y^{6}}{x y}=\frac{x^{4}-x^{2} y^{2}+y^{4}}{x y}=\frac{1}{x y}-3 x y$. The minimum value is $\frac{1}{2}$.)