a) $P = \bigg(\dfrac{4 \sqrt x}{\sqrt x + 2} - \dfrac{8x}{x - 4}\bigg) : \bigg(\dfrac{\sqrt x + 2}{\sqrt x - 2} + 3\bigg) (ĐKXĐ: x \geq 0; x \neq 1 ; x \neq 4)$
$P = \bigg(\dfrac{4\sqrt x}{\sqrt x + 2} - \dfrac{8x}{(\sqrt x +2)(\sqrt x - 2)}\bigg) : \dfrac{\sqrt x + 2 + 3}{(\sqrt x - 2)}$
$P = \dfrac{4\sqrt x(\sqrt x - 2) - 8x}{(\sqrt x +2)(\sqrt x - 2)} . \dfrac{\sqrt x - 2}{\sqrt x + 2 + 3\sqrt x - 6}$
$P = \dfrac{4x - 8\sqrt x - 8x}{(\sqrt x +2)(\sqrt x - 2)} . \dfrac{\sqrt x -2}{4\sqrt x - 4}$
$P = \dfrac{-4\sqrt x(\sqrt x + 2)}{(\sqrt x +2)(\sqrt x - 2)} . \dfrac{\sqrt x - 2}{4(\sqrt x - 1)}$
$P = \dfrac{-\sqrt x}{\sqrt x - 1}$
b) Để $P = -4$
⇔ $\dfrac{- \sqrt x}{\sqrt x - 1} = -4$
⇒ $-4(\sqrt x - 1) = -\sqrt x$
⇔ $-4\sqrt x +\sqrt x +4 = 0$
⇔ $-3\sqrt x = -4$
⇔ $\sqrt x = \dfrac{4}{3}$
⇒ $x = \dfrac{16}{9}$ (T/m)
