Đáp án:
$\displaystyle \begin{array}{{>{\displaystyle}l}} a.P=\frac{4}{\sqrt{x} +3}\\ b.P=\frac{4}{\sqrt{2020} +2} \end{array}$
Giải thích các bước giải:
$\displaystyle \begin{array}{{>{\displaystyle}l}} a.\ P=\frac{x+6\sqrt{x} +9-x+6\sqrt{x} -9}{\left(\sqrt{x} +3\right)\left(\sqrt{x} -3\right)} .\frac{\sqrt{x} -3}{3\sqrt{x}}\\ P=\frac{12\sqrt{x}}{\left(\sqrt{x} +3\right)\left(\sqrt{x} -3\right)} .\frac{\sqrt{x} -3}{3\sqrt{x}}\\ P=\frac{4}{\sqrt{x} +3}\\ b.\ x=2021-2\sqrt{2020} =\left(\sqrt{2020} -1\right)^{2}\\ \Rightarrow P=\frac{4}{\sqrt{2020} -1+3} =\frac{4}{\sqrt{2020} +2} \end{array}$
