\[\boxed{\text{646.}\text{\ }\text{еуроки}\text{-}\text{ответы}\text{\ }\text{на}\text{\ }\text{пятёрку}}\]
\[\textbf{а)}\ \frac{x - y}{\sqrt{x} - \sqrt{y}} - \sqrt{x} =\]
\[= \frac{\left( \sqrt{x} - \sqrt{y} \right)\left( \sqrt{x} + \sqrt{y} \right)}{\left( \sqrt{x} - \sqrt{y} \right)} -\]
\[- \sqrt{x} = \sqrt{x} + \sqrt{y} - \sqrt{x} = \sqrt{y}\]
\[\textbf{б)}\ \sqrt{x} - \frac{x - y}{\sqrt{x} + \sqrt{y}} = \sqrt{x} -\]
\[- \frac{\left( \sqrt{x} - \sqrt{y} \right)\left( \sqrt{x} + \sqrt{y} \right)}{\left( \sqrt{x} + \sqrt{y} \right)} =\]
\[= \sqrt{x} - \sqrt{x} + \sqrt{y} = \sqrt{y}\ \]