Đáp án:
$\displaystyle D=\frac{4a-4a\sqrt{a}}{\sqrt{a} -3}$
Giải thích các bước giải:
$\displaystyle \begin{array}{{>{\displaystyle}l}} D=\left(\frac{2+\sqrt{a}}{2-\sqrt{a}} -\frac{2-\sqrt{a}}{2+\sqrt{a}} -\frac{4a}{4-a}\right) :\left(\frac{2}{2-\sqrt{a}} -\frac{\sqrt{a} +3}{2\sqrt{a} -a}\right)\\ ĐKXĐ:\ a\geqslant 0\ và\ a\neq 4\\ D=\left(\frac{\left( 2+\sqrt{a}\right)^{2} -\left( 2-\sqrt{a}\right)^{2} -4a}{\left( 2-\sqrt{a}\right)\left( 2+\sqrt{a}\right)}\right) :\left(\frac{2\sqrt{a} -\sqrt{a} -3}{\sqrt{a}\left( 2-\sqrt{a}\right)}\right)\\ =\frac{4\sqrt{a} -4a}{\left( 2-\sqrt{a}\right)\left( 2+\sqrt{a}\right)} .\frac{\sqrt{a}\left( 2-\sqrt{a}\right)}{\sqrt{a} -3} =\frac{4a-4a\sqrt{a}}{\sqrt{a} -3} \end{array}$
