Rút gọn biều thức A=frac12-sqrt3+sqrt7-4 sqrt3

$A=\frac{1}{2-\sqrt{3}}+\sqrt{7-4 \sqrt{3}}$

$=\frac{1}{2-\sqrt{3}}+\sqrt{4-4 \sqrt{3}+3}=\frac{1}{2-\sqrt{3}}+\sqrt{(2-\sqrt{3})^{2}}$

$=\frac{1}{2-\sqrt{3}}+|2-\sqrt{3}|$

$=\frac{2+\sqrt{3}}{(2-\sqrt{3})(2+\sqrt{3})}+2-\sqrt{3}=\frac{2+\sqrt{3}}{4-3}+2-\sqrt{3}$

$=2+\sqrt{3}+2-\sqrt{3}=4$