a, đk x >= 0 ; x khác 4
\(P=\left(\dfrac{\sqrt{x}+2-2}{\left(\sqrt{x}+2\right)^2}\right):\left(\dfrac{2-\left(\sqrt{x}+2\right)}{x-4}\right)=\dfrac{\sqrt{x}\left(x-4\right)}{-\sqrt{x}\left(\sqrt{x}+2\right)^2}=-\dfrac{\sqrt{x}-2}{\sqrt{x}+2}\)
b, Ta có \(-\dfrac{\sqrt{x}-2}{\sqrt{x}+2}=-\dfrac{1}{2}\Rightarrow2\sqrt{x}-4=\sqrt{x}+2\Leftrightarrow\sqrt{x}=6\Leftrightarrow x=36\left(tm\right)\)
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