b) B = 2 + căn bậc hai 3/ căn bậc hai 7 - 4 căn bậc hai 3 - 2 - căn bậc hai 3/ căn bậc hai 7 + 4 căn bậc hai 3

* Hướng dẫn giải

b) $\mathrm{B}=\frac{2+\sqrt{3}}{\sqrt{7-4\sqrt{3}}}-\frac{2-\sqrt{3}}{\sqrt{7+4\sqrt{3}}}=\frac{2+\sqrt{3}}{\sqrt{{\left(2-\sqrt{3}\right)}^2}}-\frac{2-\sqrt{3}}{\sqrt{{\left(2+\sqrt{3}\right)}^2}}=\frac{2+\sqrt{3}}{2-\sqrt{3}}-\frac{2-\sqrt{3}}{2+\sqrt{3}}$

$$\begin{gathered} =\frac{{\left(2+\sqrt{3}\right)}^2}{\left(2-\sqrt{3}\right)\left(2+\sqrt{3}\right)}-\frac{{\left(2-\sqrt{3}\right)}^2}{\left(2+\sqrt{3}\right)\left(2-\sqrt{3}\right)} \\ ={\left(2+\sqrt{3}\right)}^2-{\left(2-\sqrt{3}\right)}^2 \\ =\left(2+\sqrt{3}+2-\sqrt{3}\right)\left(2+\sqrt{3}-2+\sqrt{3}\right) \\ =4.2\sqrt{3}=8\sqrt{3} \\ \mathrm{Vậy} \mathrm{B}=8\sqrt{3} \end{gathered}$$