\(\sqrt{15-10\sqrt{2}}\\ =\sqrt{15-2\cdot\sqrt{5}\cdot\sqrt{5}\cdot\sqrt{2}}\\ =\sqrt{\left(\sqrt{10}\right)^2-2\cdot\sqrt{10}\cdot\sqrt{5}+\left(\sqrt{5}\right)^2}\\ =\sqrt{\left(\sqrt{10}-\sqrt{5}\right)^2}\\ =\sqrt{10}-\sqrt{5}\\ =\sqrt{5}\left(\sqrt{2}-1\right)\)