8. \(\sqrt{20}+2\sqrt{45}-3\sqrt{500}\)

Упростим корни: \(\sqrt{20}=2\sqrt{5}\), \(\sqrt{45}=3\sqrt{5}\), \(\sqrt{500}=10\sqrt{5}\).

\(2\sqrt{5}+2\cdot3\sqrt{5}-3\cdot10\sqrt{5}=2\sqrt{5}+6\sqrt{5}-30\sqrt{5}=-22\sqrt{5}\).

Ответ: \(-22\sqrt{5}\).