Câu 3.
$A = \sqrt{6 - 2\sqrt{5}} - \sqrt{6 - 2\sqrt{5}} = 0$
Câu 4.
$\sqrt{122^2 - 22^2} = \sqrt{(122 - 22)(122 + 22} = \sqrt{100}.\sqrt{144} = 10.12 = 120$
Câu 5.
$(2 + \sqrt{3}) : \sqrt{7 - 4\sqrt{3}} =$
$= (2 + \sqrt{3}) : \sqrt{2^2 - 2.2\sqrt{3} + (\sqrt{3})^2} =$
$= (2 + \sqrt{3}) : \sqrt{2 - \sqrt{3})^2} =$
$= (2 + \sqrt{3}) : (2 - \sqrt{3}) = \dfrac{2 + \sqrt{3}}{2 - \sqrt{3}}$
$= \dfrac{(2 + \sqrt{3})(2 + \sqrt{3})}{(2 - \sqrt{3})(2 + \sqrt{3})} = 7 + 4\sqrt{3}$
