8. (\(\sqrt{20} - \sqrt{5}\)) \(\cdot \sqrt{5}\)

8. (\(\sqrt{20} - \sqrt{5}\)) \(\cdot \sqrt{5} = (\sqrt{4 \cdot 5} - \sqrt{5}\)) \(\cdot \sqrt{5} = (2\sqrt{5} - \sqrt{5}\)) \(\cdot \sqrt{5} = \sqrt{5} \cdot \sqrt{5} = 5\)

Ответ: 5