Tính √(21 + 6√6) + √(21 - 6√6)

tính:
$$\begin{gathered} \sqrt{21+6\sqrt{6}}+\sqrt{21-6\sqrt{6}} \\ \sqrt{7-4\sqrt{3}}-\sqrt{7+4\sqrt{3}} \\ \sqrt{4+\sqrt{7}}-\sqrt{4-\sqrt{7}} \\ \sqrt{4-\sqrt{10-2\sqrt{5}}}-\sqrt{4+\sqrt{10-2\sqrt{5}}} \end{gathered}$$