20) \( \sqrt{72} - \sqrt{8} - 4\sqrt{2} - 13 \)

\( \sqrt{72} = 6\sqrt{2}, \sqrt{8} = 2\sqrt{2} \). Тогда \( 6\sqrt{2} - 2\sqrt{2} - 4\sqrt{2} - 13 = 0 - 13 = -13 \). Ответ: \(-13\).