a: \(F=\left(\dfrac{1}{x+3\sqrt{x}}-\dfrac{1}{\sqrt{x}+3}\right):\dfrac{1-\sqrt{x}}{x+6\sqrt{x}+9}\)
\(=\left(\dfrac{1}{\sqrt{x}\left(\sqrt{x}+3\right)}-\dfrac{1}{\sqrt{x}+3}\right)\cdot\dfrac{\left(x+6\sqrt{x}+9\right)}{1-\sqrt{x}}\)
\(=\dfrac{1-\sqrt{x}}{\sqrt{x}\left(\sqrt{x}+3\right)}\cdot\dfrac{\left(\sqrt{x}+3\right)^2}{1-\sqrt{x}}=\dfrac{\sqrt{x}+3}{\sqrt{x}}\)
b: \(F=\dfrac{5}{2}\)
=>\(\dfrac{\sqrt{x}+3}{\sqrt{x}}=\dfrac{5}{2}\)
=>\(5\sqrt{x}=2\sqrt{x}+6\)
=>\(3\sqrt{x}=6\)
=>\(\sqrt{x}=2\)
=>x=4(nhận)
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