a: \(A=\dfrac{1}{\sqrt{6}-\sqrt{5}}-\sqrt{5}\)
\(=\dfrac{\sqrt{6}+\sqrt{5}}{6-5}-\sqrt{5}\)
\(=\sqrt{6}+\sqrt{5}-\sqrt{5}=\sqrt{6}\)
b: \(B=\left(\dfrac{3x-3\sqrt{x}}{x-1}-\dfrac{\sqrt{x}}{x+\sqrt{x}}\right):\dfrac{3\sqrt{x}-1}{x-1}\)
\(=\left(\dfrac{3\sqrt{x}\left(\sqrt{x}-1\right)}{\left(\sqrt{x}-1\right)\left(\sqrt{x}+1\right)}-\dfrac{\sqrt{x}}{\sqrt{x}\left(\sqrt{x}+1\right)}\right)\cdot\dfrac{x-1}{3\sqrt{x}-1}\)
\(=\left(\dfrac{3\sqrt{x}}{\sqrt{x}+1}-\dfrac{1}{\sqrt{x}+1}\right)\cdot\dfrac{x-1}{3\sqrt{x}-1}\)
\(=\dfrac{3\sqrt{x}-1}{\sqrt{x}+1}\cdot\dfrac{x-1}{3\sqrt{x}-1}=\dfrac{x-1}{\sqrt{x}+1}=\sqrt{x}-1\)
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