Đáp án:
`#Nan`
`1)A=(\sqrt{4}-1)/(\sqrt{4}-3)`
`=(2-1)/(2-3)`
`=1/-1`
`=-1`
`2)B=((\sqrt{x}-5)/(x-9)+1/(\sqrt{x}-3)):1/(\sqrt{x}-3)`
`=((\sqrt{x}-5)/((\sqrt{x}-3)(\sqrt{x}+3))+(\sqrt{x}+3)/((\sqrt{x}-3)(\sqrt{x}+3))):1/(\sqrt{x}-3)`
`=(\sqrt{x}-5+\sqrt{x}+3)/((\sqrt{x}-3)(\sqrt{x}+3)).(\sqrt{x}-3)/1`
`=(\sqrt{x}-5+\sqrt{x}+3)/((\sqrt{x}-3)(\sqrt{x}+3)).(\sqrt{x}-3)/1`
`=(2\sqrt{x}-2)/(\sqrt{x}+3)`
`3)\sqrt{x}AB >= 0`
$=>\sqrt{x}.(-1).(2\sqrt{x}-2)/(\sqrt{x}+3)>=0$
$=>(-\sqrt{x}(2\sqrt{x}-2))/(\sqrt{x}+3)>=0$
$=>(-2x+4\sqrt{x})/(\sqrt{x}+3)ge0$
$\left[ \begin{array}{l}-2x+4\sqrt{x}\ \le 0\\\sqrt{x}+3\>=0\end{array} \right.$
$\left[ \begin{array}{l}x\>= 4\\x\>=0\end{array} \right.$
$\left[ \begin{array}{l}-2x+4\sqrt{x}\ \ge 0\\\sqrt{x}+3\le0\end{array} \right.$
$\left[ \begin{array}{l}x\le4\\x=∅\end{array} \right.$
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