Упростите:

а) $$ \frac{\sqrt{3}-3}{\sqrt{5}-\sqrt{15}} = \frac{\sqrt{3}(1-\sqrt{3})}{\sqrt{5}(1-\sqrt{3})} = \frac{\sqrt{3}}{\sqrt{5}} = \frac{\sqrt{15}}{5} $$

Ответ: $$ \frac{\sqrt{15}}{5} $$