` ĐKXĐ: x > 4 `
` F = (\frac{\sqrt{x} - 2}{\sqrt{x} + 2} - \frac{\sqrt{x} + 2}{\sqrt{x} - 2})(\sqrt{x} - \frac{4}{\sqrt{x}}) `
` = \frac{(\sqrt{x} - 2)^2 - (\sqrt{x} + 2)^2}{(\sqrt{x} + 2)(\sqrt{x} - 2)} . \frac{x - 4}{\sqrt{x}} `
` = \frac{(\sqrt{x} - 2 - \sqrt{x} - 2)(\sqrt{x} - 2 + \sqrt{x} + 2)}{(\sqrt{x} + 2)(\sqrt{x} - 2)} . \frac{(\sqrt{x} - 2)(\sqrt{x} + 2)}{\sqrt{x}} `
` = \frac{-4.2\sqrt{x}}{1} . 1/(\sqrt{x}) `
` = -8 `
