\(5f(x) + 4f(\frac 1x) = x^2 - 4 \) and \(y = 9f(x).x^2\). If y is strictly increasing function, find interval of x.
(1) \(\left(-\infty, \frac{-1}{\sqrt 5}\right] \cup \left( \frac{-1}{\sqrt 5}, 0\right)\)
(2) \( \left( \frac{-1}{\sqrt 5}, 0\right) \cup \left(0, \frac{1}{\sqrt 5}\right)\)
(3) \( \left(0, \frac{1}{\sqrt 5}\right) \cup \left( \frac{1}{\sqrt 5}, \infty\right)\)
(4) \(\left(-\sqrt{\frac 25}, 0 \right)\cup \left(\sqrt \frac25, \infty\right)\)
