Do not use a calculator in this question. In triangle \(ABC\), \(AB=2\sqrt5-1\), \(BC=2+\sqrt5\), and angle \(ABC=90^\circ\).

(i) Find the exact length of \(AC\).

(ii) Find \(\tan ACB\), giving your answer in the form \(p+q\sqrt r\), where \(p\), \(q\) and \(r\) are integers.

(iii) Hence find \(\operatorname{sec}^2ACB\), giving your answer in the form \(s+t\sqrt u\), where \(s\), \(t\) and \(u\) are integers.