a) $$\frac{5 + \sqrt{a}}{25 - a} = \frac{5 + \sqrt{a}}{(5 - \sqrt{a})(5 + \sqrt{a})} = \frac{1}{5 - \sqrt{a}}$$
б) $$\frac{7 + \sqrt{7}}{\sqrt{14} + \sqrt{2}} = \frac{7 + \sqrt{7}}{\sqrt{2}(\sqrt{7} + 1)} = \frac{\sqrt{7}(\sqrt{7} + 1)}{\sqrt{2}(\sqrt{7} + 1)} = \frac{\sqrt{7}}{\sqrt{2}} = \frac{\sqrt{14}}{2}$$
Ответ:
a) $$\boxed{\frac{1}{5 - \sqrt{a}}}$$
б) $$\boxed{\frac{\sqrt{14}}{2}}$$