a/ ĐKXĐ: \(x\ne 0;3\)
\(A=(\dfrac{3}{x-3\sqrt x}+\dfrac{1}{\sqrt x+3}).\dfrac{x-9}{\sqrt x}\\=(\dfrac{3(\sqrt x+3)}{\sqrt x(\sqrt x-3)(\sqrt x+3)}+\dfrac{\sqrt x(\sqrt x-3)}{\sqrt x(\sqrt x+3)(\sqrt x-3)}).\dfrac{x-9}{\sqrt x}\\=\dfrac{3\sqrt x+9+x-3\sqrt x}{\sqrt x(x-9)}.\dfrac{x-9}{\sqrt x}\\=\dfrac{x+9}{\sqrt x}.\dfrac{1}{\sqrt x}=\dfrac{x+9}{x}\)
b/ \(\sqrt 5.(\dfrac{1}{\sqrt 5-2}+\dfrac{1}{\sqrt 5+2})\\=\sqrt 5.(\dfrac{\sqrt 5+2+\sqrt 5-2}{(\sqrt 5-2)(\sqrt 5-2)}\\=\sqrt 5.\dfrac{2\sqrt 5}{1}\\=\sqrt 5.2\sqrt 5=2.5=10\)
