(1) If real numbers $x, y$ satisfy $\frac{x+y}{1-xy}=\sqrt{5}$, then $\frac{|1-xy|}{\sqrt{1+x^{2}} \sqrt{1+y^{2}}}=$
(1) $\frac{\sqrt{6}}{6}$ Hint: Let $x=\tan \alpha, y=\tan \beta$, then $\tan (\alpha+\beta)=\sqrt{5}$, and
$$
\frac{|1-x y|}{\sqrt{1+x^{2}} \sqrt{1+y^{2}}}=\left|\frac{1-\tan \alpha \tan \beta}{\cos ^{-1} \alpha \cdot \cos ^{-1} \beta}\right|=|\cos (\alpha+\beta)|=\frac{\sqrt{6}}{6} .
$$