\(a,ĐK:x\ge0;x\ne9\\ A=\dfrac{x+3\sqrt{x}-\sqrt{x}+3-6}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\dfrac{x+2\sqrt{x}-3}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}\\ =\dfrac{\left(\sqrt{x}+3\right)\left(\sqrt{x}-1\right)}{\left(\sqrt{x}-3\right)\left(\sqrt{x}+3\right)}=\dfrac{\sqrt{x}-1}{\sqrt{x}-3}\)
\(b,x=\sqrt{14-6\sqrt{5}}+\sqrt{6+2\sqrt{5}}=\sqrt{\left(3-\sqrt{5}\right)^2}+\sqrt{\left(\sqrt{5}+1\right)^2}\\ =3-\sqrt{5}+\sqrt{5}+1=4\\ \Leftrightarrow A=\dfrac{2-1}{2-3}=\dfrac{1}{-1}=-1\)
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