$\displaystyle \begin{array}{{>{\displaystyle}l}} \mathrm{a\ ĐK:\ x\geqslant 0,\ x\neq 4}\\ \mathrm{A=\frac{x}{x-4} +\frac{\sqrt{x} +2}{x-4} +\frac{\sqrt{x} -2}{x-4}}\\ \mathrm{A=\frac{x+2\sqrt{x}}{\left(\sqrt{x} -2\right)\left(\sqrt{x} +2\right)} =\frac{\left(\sqrt{x} +2\right)\sqrt{x}}{\left(\sqrt{x} -2\right)\left(\sqrt{x} +2\right)}}\\ \mathrm{A=\frac{\sqrt{x}}{\left(\sqrt{x} -2\right)}}\\ \mathrm{b.\ x=4-2\sqrt{3} =\left(\sqrt{3} -1\right)^{2} ,\ thay\ vào\ A,\ ta\ có:}\\ \mathrm{A=\frac{\sqrt{3} -1}{\sqrt{3} -1-2} =\frac{\sqrt{3} -1}{\sqrt{3} -3}}\\ \mathrm{c.\ A=-\frac{1}{3}}\\ \mathrm{\Leftrightarrow 3\sqrt{x} +\sqrt{x} -2=0}\\ \mathrm{\Leftrightarrow 4\sqrt{x} -2=0}\\ \mathrm{\Leftrightarrow \sqrt{x} =\frac{1}{2}}\\ \mathrm{\Leftrightarrow x=\frac{1}{4} \ ( TM)}\\ \mathrm{vậy\ \ x=\frac{1}{4} \ là\ gtct} \end{array}$
