1: Khi x=1,44 thì \(A=\dfrac{1.44+7}{1.2}=\dfrac{8.44}{1.2}=\dfrac{211}{30}\)
2: \(B=\dfrac{\sqrt{x}}{\sqrt{x}+3}+\dfrac{2\sqrt{x}-1}{\sqrt{x}-3}-\dfrac{2x-\sqrt{x}-3}{x-9}\)
\(=\dfrac{\sqrt{x}\left(\sqrt{x}-3\right)+\left(2\sqrt{x}-1\right)\left(\sqrt{x}+3\right)-2x+\sqrt{x}+3}{x-9}\)
\(=\dfrac{x-3\sqrt{x}+2x+5\sqrt{x}-3-2x+\sqrt{x}+3}{x-9}\)
\(=\dfrac{x+3\sqrt{x}}{x-9}=\dfrac{\sqrt{x}}{\sqrt{x}-3}\)
3: \(S=\dfrac{1}{B}+A=\dfrac{\sqrt{x}-3}{\sqrt{x}}+\dfrac{x+7}{\sqrt{x}}=\dfrac{x+\sqrt{x}+4}{\sqrt{x}}\)
\(=\sqrt{x}+1+\dfrac{4}{\sqrt{x}}\)
\(\Leftrightarrow S>=2\cdot\sqrt{\sqrt{x}\cdot\dfrac{4}{\sqrt{x}}}+1=4+1=5\)
Dấu = xảy ra khi \(\left(\sqrt{x}\right)^2=4\)
=>x=4
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