8.1) \(\dfrac{\sqrt{21}\cdot\sqrt{14}}{\sqrt6}\); 8.2) \(\dfrac{\sqrt{32}\cdot\sqrt6}{\sqrt{12}}\); 8.3) \(\dfrac{\sqrt{35}\cdot\sqrt{21}}{\sqrt{15}}\); 8.4) \(\dfrac{\sqrt{20}\cdot\sqrt{32}}{\sqrt{10}}\); 8.5) \(\dfrac{\sqrt{22}\cdot\sqrt{33}}{\sqrt6}\).

Объединяем под одним корнем: \(\dfrac{\sqrt a\cdot\sqrt b}{\sqrt c}=\sqrt{\dfrac{ab}{c}}\).

1. 8.1: \(\sqrt{21\cdot14/6}=\sqrt{49}=7\).
2. 8.2: \(\sqrt{32\cdot6/12}=\sqrt{16}=4\).
3. 8.3: \(\sqrt{35\cdot21/15}=\sqrt{49}=7\).
4. 8.4: \(\sqrt{20\cdot32/10}=\sqrt{64}=8\).
5. 8.5: \(\sqrt{22\cdot33/6}=\sqrt{121}=11\).

Ответ: 8.1) 7; 8.2) 4; 8.3) 7; 8.4) 8; 8.5) 11.