Rút gọn biều thức : B=frac2sqrt7-sqrt6-sqrt28+sqrt54

$B=\frac{2}{\sqrt{7}-\sqrt{6}}-\sqrt{28}+\sqrt{54}$

$=\frac{2(\sqrt{7}+\sqrt{6})}{(\sqrt{7}-\sqrt{6})(\sqrt{7}+\sqrt{6})}-\sqrt{2^{2} \cdot 7}+\sqrt{3^{2} \cdot 6}$

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

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

$=2 \sqrt{6}+3 \sqrt{6}=(2+3) \sqrt{6}=5 \sqrt{6}$