[tex]\frac{x^{2} -y^{2}-2y-\sqrt{x^{4}}+\sqrt{y^{4} } }{(\sqrt{x} -\sqrt{y})(x-y) } =\frac{x^{2} -y^{2}-2y-x^{2}+y^{2} }{(x-y)(\sqrt{x} -\sqrt{y} )} =-\frac{2y}{(x-y)(\sqrt{x}-\sqrt{y} ) }[/tex]
