\(1,A=\dfrac{5+1}{25-9}=\dfrac{6}{16}=\dfrac{3}{8}\\ 2,B=\dfrac{x-\sqrt{x}-12-x+3\sqrt{x}+18}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\dfrac{2\left(\sqrt{x}+3\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\dfrac{2}{\sqrt{x}-3}\\ 3,P=2A:B=\dfrac{2\left(\sqrt{x}+1\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\cdot\dfrac{\sqrt{x}-3}{2}=\dfrac{\sqrt{x}+1}{\sqrt{x}+3}\\ P=1-\dfrac{2}{\sqrt{x}+3}\)
P đạt min \(\Leftrightarrow\dfrac{2}{\sqrt{x}+3}\) đạt max \(\Leftrightarrow\sqrt{x}+3\) đạt min \(\Leftrightarrow\sqrt{x}+3=1+3=4\left(x_{min}\text{ nguyên dương}\right)\)
Vậy \(P_{min}=\dfrac{1+1}{1+3}=\dfrac{1}{2}\Leftrightarrow x=1\)
