Rút gọn \(D=\left(\frac{5+\...

C

- Hướng dẫn giải

 \(D=\left(\frac{5+\sqrt{5}}{5-\sqrt{5}}+\frac{5-\sqrt{5}}{5+\sqrt{5}}\right): \frac{1}{\sqrt{7-4 \sqrt{3}}}\)

\(\begin{array}{l} =\frac{(5-\sqrt{5})^{2}+(5+\sqrt{5})^{2}}{(5-\sqrt{5})(5+\sqrt{5})} \sqrt{7-4 \sqrt{3}} \\ =\frac{(5-\sqrt{5})^{2}+(5+\sqrt{5})^{2}}{(5-\sqrt{5})(5+\sqrt{5})} \sqrt{(2-\sqrt{3})^{2}} \\ =3|2-\sqrt{3}| \\ =3(2-\sqrt{3}) \\ =6-3 \sqrt{3} \end{array}\)