b) B = 2/ căn bậc hai 7 - căn bậc hai 6 - căn bậc hai 28 + căn bậc hai 54

* Hướng dẫn giải

b) $\mathrm{B}=\frac{2}{\sqrt{7}-\sqrt{6}}-\sqrt{28}+\sqrt{54}=\frac{2\left(\sqrt{7}+\sqrt{6}\right)}{\left(\sqrt{7}-\sqrt{6}\right)\left(\sqrt{7}+\sqrt{6}\right)}-\sqrt{7.4}+\sqrt{9.6}=\frac{2\sqrt{7}+2\sqrt{6}}{7-6}-2\sqrt{7}+3\sqrt{6}$

$=2\sqrt{7}+2\sqrt{6}-2\sqrt{7}+3\sqrt{6}=5\sqrt{6}$

$\mathrm{Vậy} \mathrm{B}=5\sqrt{6}$