$\sqrt{9 - 4\sqrt{5}} = \sqrt{4 + 5 - 4\sqrt{5}} = \sqrt{2^2 - 2 * 2 * \sqrt{5} + (\sqrt{5})^2} = \sqrt{(2 - \sqrt{5})^2} = |2 - \sqrt{5}| = -2 + \sqrt{5} = \sqrt{5} - 2$

$\sqrt{7 - 2\sqrt{10}} = \sqrt{5 + 2 - 2\sqrt{10}} = \sqrt{(\sqrt{5})^2 - 2\sqrt{5 * 2} + (\sqrt{2})^2} = \sqrt{(\sqrt{5} - \sqrt{2})^2} = |\sqrt{5} - \sqrt{2}| = \sqrt{5} - \sqrt{2}$

$\sqrt{37 - 20\sqrt{3}} = \sqrt{25+ 12 - 20\sqrt{3}} = \sqrt{5^2 - 2 * 5 * 2\sqrt{3} + (2\sqrt{3})^2} = \sqrt{(5 - 2\sqrt{3})^2} = |5 - 2\sqrt{3}| = 5 - 2\sqrt{3}$