$P=\left( \dfrac{\sqrt{x}}{\sqrt{x}-2}+ \dfrac{\sqrt{x}}{\sqrt{x}+2} \right). \dfrac{4-x}{\sqrt{4x}}\\ =\left( \dfrac{\sqrt{x}(\sqrt{x}+2)}{(\sqrt{x}-2)(\sqrt{x}+2)}+ \dfrac{\sqrt{x}(\sqrt{x}-2)}{(\sqrt{x}-2)(\sqrt{x}+2)} \right). \dfrac{-(\sqrt{x}-2)(\sqrt{x}+2)}{2\sqrt{x}}\\ =\dfrac{\sqrt{x}(\sqrt{x}+2)+\sqrt{x}(\sqrt{x}-2)}{(\sqrt{x}-2)(\sqrt{x}+2)}. \dfrac{-(\sqrt{x}-2)(\sqrt{x}+2)}{2\sqrt{x}}\\ =\dfrac{2x}{(\sqrt{x}-2)(\sqrt{x}+2)}. \dfrac{-(\sqrt{x}-2)(\sqrt{x}+2)}{2\sqrt{x}}\\ =-\sqrt{x}\\ b)P=-\sqrt{x} < 0 \, \, \forall \, \, x>0$
$=>$Không tồn tại x thoả mãn $P>3$
$2)Q=\left( \dfrac{\sqrt{x}}{\sqrt{x}-1}+ \dfrac{\sqrt{x}}{\sqrt{x}+1} \right)+\dfrac{3-\sqrt{x}}{x-1}\\ = \dfrac{\sqrt{x}(\sqrt{x}+1)}{(\sqrt{x}-1)(\sqrt{x}+1)}+ \dfrac{\sqrt{x}(\sqrt{x}-1)}{(\sqrt{x}-1)(\sqrt{x}+1)} +\dfrac{3-\sqrt{x}}{(\sqrt{x}-1)(\sqrt{x}+1)}\\ =\dfrac{\sqrt{x}(\sqrt{x}+1)+\sqrt{x}(\sqrt{x}-1)+3-\sqrt{x}}{(\sqrt{x}-1)(\sqrt{x}+1)}\\ =\dfrac{2x-\sqrt{x}+3}{x-1}\\ b)Q=-1<=>2x-\sqrt{x}+3=1-x\\ <=>3x-\sqrt{x}+2=0\\ <=>(x-\sqrt{x}+\dfrac{1}{4})+2x+\dfrac{7}{4}=0\\ <=>\underbrace{{(\sqrt{x}-\dfrac{1}{2})^2+2x+\dfrac{7}{4}}}_{>0 \, \, \forall x\ge 0}=0$
$=>$Không tồn tại x thoả mãn $Q=-1$
