Решение:

a) $$\frac{\sqrt{10}+5}{2+\sqrt{10}} = \frac{\sqrt{10}+5}{2+\sqrt{10}} = \frac{\sqrt{10}+5}{2+\sqrt{10}} \cdot \frac{2-\sqrt{10}}{2-\sqrt{10}} = \frac{(\sqrt{10}+5)(2-\sqrt{10})}{(2+\sqrt{10})(2-\sqrt{10})} = \frac{2\sqrt{10} - 10 + 10 - 5\sqrt{10}}{4 - 10} = \frac{-3\sqrt{10}}{-6} = \frac{\sqrt{10}}{2}$$

Ответ: $$\frac{\sqrt{10}}{2}$$

б) $$\frac{x-3\sqrt{x}}{2\sqrt{x}-6} = \frac{\sqrt{x}(\sqrt{x}-3)}{2(\sqrt{x}-3)} = \frac{\sqrt{x}}{2}$$

Ответ: $$\frac{\sqrt{x}}{2}$$