a: Thay x=9 vào A, ta được:
\(A=\dfrac{3}{3-4}=\dfrac{3}{-1}=-3\)
b: Đặt \(P=\dfrac{B}{A}\)
\(=\left(\dfrac{x+4}{x-16}+\dfrac{1}{\sqrt{x}+4}\right):\dfrac{\sqrt{x}}{\sqrt{x}-4}\)
\(=\left(\dfrac{x+4}{\left(\sqrt{x}-4\right)\left(\sqrt{x}+4\right)}+\dfrac{1}{\sqrt{x}+4}\right)\cdot\dfrac{\sqrt{x}-4}{\sqrt{x}}\)
\(=\dfrac{x+4+\sqrt{x}-4}{\left(\sqrt{x}-4\right)\left(\sqrt{x}+4\right)}\cdot\dfrac{\sqrt{x}-4}{\sqrt{x}}\)
\(=\dfrac{x+\sqrt{x}}{\sqrt{x}}\cdot\dfrac{1}{\sqrt{x}+4}=\dfrac{\sqrt{x}+1}{\sqrt{x}+4}\)
c: \(P-2=\dfrac{\sqrt{x}+1-2\left(\sqrt{x}+4\right)}{\sqrt{x}+4}=\dfrac{-\sqrt{x}-7}{\sqrt{x}+4}< 0\)
=>P<2
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