а) $$\frac{5-\sqrt{5}}{\sqrt{30} - \sqrt{6}} = \frac{\sqrt{5}(\sqrt{5} - 1)}{\sqrt{6}(\sqrt{5} - 1)} = \frac{\sqrt{5}}{\sqrt{6}} = \frac{\sqrt{30}}{6}$$
б) $$\frac{a-64}{8+\sqrt{a}} = \frac{(\sqrt{a}-8)(\sqrt{a}+8)}{8+\sqrt{a}} = \sqrt{a}-8$$
Ответ: а) $$\frac{\sqrt{30}}{6}$$; б) $$\sqrt{a}-8$$