Đáp án:
$-\sqrt{6}$
Giải thích các bước giải:
$\sqrt{4-\sqrt{15}} -\sqrt{4+\sqrt{15}}$
$=\sqrt{(8-2\sqrt{15}):2} -\sqrt{(8+2\sqrt{15}):2}$
$=\sqrt{(5-2\sqrt{5}.\sqrt{3}+3):2} -\sqrt{(5+2\sqrt{5}.\sqrt{3}+3):2}$
$=\sqrt{(\sqrt{5}-\sqrt{3})^2:2} -\sqrt{(\sqrt{5}+\sqrt{3})^2:2}$
$=\dfrac{\sqrt{5}-\sqrt{3}}{\sqrt{2}} -\dfrac{\sqrt{5}+\sqrt{3}}{\sqrt{2}}$
$=\dfrac{-2\sqrt{3}}{\sqrt{2}} $
$=-\sqrt{6}$
