a) $C=(\dfrac1{\sqrt x-1}-\dfrac{2\sqrt x}{x\sqrt x-x+\sqrt x-1}): (\dfrac{\sqrt x+x}{x\sqrt x+x+\sqrt x+1}+\dfrac1{x+1})$
$=\left [ \dfrac1{\sqrt x-1}-\dfrac{2\sqrt x}{(\sqrt x-1)(x+1)} \right ]:\left [ \dfrac{\sqrt x(\sqrt x+1)}{(\sqrt x+1)(x+1)}+\dfrac1{x+1} \right ]$
$=\dfrac{x+1-2\sqrt x}{(\sqrt x-1)(x+1)}:\dfrac{\sqrt x+1}{x+1}$
$=\dfrac{(\sqrt x-1)^2}{(\sqrt x-1)(x+1)}.\dfrac{x+1}{\sqrt x+1}$
$=\dfrac{\sqrt x-1}{\sqrt x+1}$
b) $(\sqrt{x}+1).C=m-x$
$\Leftrightarrow \sqrt{x}-1=m-x$
$\Leftrightarrow x+\sqrt{x}+\dfrac14-\dfrac 54=m$
$\Leftrightarrow (\sqrt x+\dfrac12)^2=m+\dfrac 54$
Mà $(\sqrt x+\dfrac12)^2\ge \dfrac14\Rightarrow m\ge -1$
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