Answer: A
Let \(x = 9 + 4\sqrt{5}\) = \(5 + 4 + 4\sqrt{5}\) = \((\sqrt{5})^2 + (2)^2 + 2 \cdot \sqrt{5} \cdot 2\) = \((\sqrt{5} + 2)^2\) ∴ \(\sqrt{x} = \sqrt{5} + 2\) ∴ \(\frac{1}{\sqrt{x}} = \frac{1}{\sqrt{5} + 2}\) = \(\frac{\sqrt{5} - 2}{(\sqrt{5} + 2)(\sqrt{5} - 2)}\) = \(\frac{\sqrt{5} - 2}{5 - 4}\) = \(\sqrt{5} - 2\) ∴ \(\sqrt{x} - \frac{1}{\sqrt{x}} = (\sqrt{5} + 2) - (\sqrt{5} - 2)\) = \(\sqrt{5} + 2 - \sqrt{5} + 2 = 4\)
Let \(x = 9 + 4\sqrt{5}\) = \(5 + 4 + 4\sqrt{5}\) = \((\sqrt{5})^2 + (2)^2 + 2 \cdot \sqrt{5} \cdot 2\) = \((\sqrt{5} + 2)^2\) ∴ \(\sqrt{x} = \sqrt{5} + 2\) ∴ \(\frac{1}{\sqrt{x}} = \frac{1}{\sqrt{5} + 2}\) = \(\frac{\sqrt{5} - 2}{(\sqrt{5} + 2)(\sqrt{5} - 2)}\) = \(\frac{\sqrt{5} - 2}{5 - 4}\) = \(\sqrt{5} - 2\) ∴ \(\sqrt{x} - \frac{1}{\sqrt{x}} = (\sqrt{5} + 2) - (\sqrt{5} - 2)\) = \(\sqrt{5} + 2 - \sqrt{5} + 2 = 4\)