`\color{#FF0000}{T}\color{#FF7F24}{i}\color{#FFFF00}{t}\color{#7FFF00}{u}\color{#00F5FF}{u}\color{#FF1493}{u}`

$a)$ `A=((\sqrt{a})/(\sqrt{a}+1)-(\sqrt{a})/(\sqrt{a}-1)+(2\sqrt{a}-4)/(a-1)):(1)/(\sqrt{a}+1) (0<=a!=1)`

`=(\sqrt{a}(\sqrt{a}-1)-\sqrt{a}(\sqrt{a}+1)+2\sqrt{a}-4)/((\sqrt{a}-1)(\sqrt{a}+1)).(\sqrt{a}+1)`

`=(a-\sqrt{a}-a-\sqrt{a}+2\sqrt{a}-4)/(\sqrt{a}-1)`

`=(4)/(1-\sqrt{a})`

Vậy `A=(4)/(1-\sqrt{a})` với `0<=a!=1`

$b)$ Có `a=4-2\sqrt{3}=(\sqrt{3}-1)^2 (TM)`

`-> \sqrt{a}=\sqrt{3}-1`

`-> A=(4)/(1-\sqrt{3}+1)=8+4\sqrt{3}`

Vậy `A=8+4\sqrt{3}` khi `a=4-2\sqrt{3}`