a) $$\frac{\sqrt{70} - \sqrt{30}}{\sqrt{35} - \sqrt{15}} = \frac{\sqrt{2} \cdot \sqrt{35} - \sqrt{2} \cdot \sqrt{15}}{\sqrt{35} - \sqrt{15}} = \frac{\sqrt{2}(\sqrt{35} - \sqrt{15})}{\sqrt{35} - \sqrt{15}} = \sqrt{2}$$
Ответ: $$\sqrt{2}$$