`a)P=(\frac{\sqrt{x}}{\sqrt{x}-2}+\frac{\sqrt{x}}{\sqrt{x}+2}).\frac{x-4}{\sqrt{4x}}(x>0,x\ne4)`
`=\frac{\sqrt{x}(\sqrt{x}+2)+\sqrt{x}(\sqrt{x}-2)}{x-4}.\frac{x-4}{2\sqrt{x}}`
`=\frac{x+2\sqrt{x}+x-2\sqrt{x}}{x-4}.\frac{x-4}{2\sqrt{x}}`
`=\frac{2x}{2\sqrt{x}}`
`=\sqrt{x}`
`b)x=7+4\sqrt{3}`
`->\sqrt{x}=\sqrt{7+4\sqrt{3}}=\sqrt{(2+\sqrt{3})^2}=2+\sqrt{3}`
Vậy `P=2+\sqrt{3}` khi `x=7+4\sqrt{3}`
`c)P>3`
`<=>\sqrt{x}>3`
`<=>x>9`
Vậy với `x>9` thì `P>3`