`***`Lời giải`***`
a)
ĐKXĐ:$\begin{cases} a-4\neq0\\a≥0\end{cases}$ `<=>`$\begin{cases} a\neq4\\a≥0\end{cases}$
b)
`A=(\frac{\sqrt{a}}{\sqrt{a}+2}-\frac{\sqrt{a}}{\sqrt{a}-2}+\frac{4\sqrt{a}-1}{a-4}):\frac{1}{a-4}`
`=\frac{\sqrt{a}(\sqrt{a}-2)-\sqrt{a}(\sqrt{a}+2)+4\sqrt{a}-1}{a-4}:\frac{1}{a-4}`
`=\frac{a-2\sqrt{a}-a-2\sqrt{a}+4\sqrt{a}-1}{a-4}:\frac{1}{a-4}`
`=\frac{-1}{a-4}:\frac{1}{a-4}`
`=\frac{-1}{a-4}.(a-4)`
`=-1`
