giúp mình với huhu khó qa

Giải thích các bước giải:

a.Ta có:
$A=\dfrac{x-7}{x-3\sqrt{x}}-\dfrac{3+\sqrt{x}}{\sqrt{x}}$

$\to A=\dfrac{x-7}{\sqrt{x}(\sqrt{x}-3)}-\dfrac{(3+\sqrt{x})(\sqrt{x}-3)}{\sqrt{x}(\sqrt{x}-3)}$ 

$\to A=\dfrac{x-7-(3+\sqrt{x})(\sqrt{x}-3)}{\sqrt{x}(\sqrt{x}-3)}$ 

$\to A=\dfrac{x-7-(x-9)}{\sqrt{x}(\sqrt{x}-3)}$ 

$\to A=\dfrac{2}{\sqrt{x}(\sqrt{x}-3)}$

b.Ta có:

$A:\dfrac1{\sqrt{x}-3}=\dfrac2{\sqrt{x}(\sqrt{x}-3)}:\dfrac1{\sqrt{x}-3}=\dfrac2{\sqrt{x}}$

Ta có:

$x=\dfrac2{10-3\sqrt{11}}=\dfrac{2\left(10+3\sqrt{11}\right)}{\left(10-3\sqrt{11}\right)\left(10+3\sqrt{11}\right)}=20+6\sqrt{11}$

$\to x=11+6\sqrt{11}+9$

$\to x=(\sqrt{11}+3)^2$

$\to \sqrt{x}=\sqrt{11}+3$

$\to A:\dfrac1{\sqrt{x}-3}=\dfrac2{\sqrt{11}+3}$