A closed cylinder has base radius \(r\), height \(h\), and volume \(V\). The total surface area of the cylinder is \(600\pi\), and \(V\), \(r\) and \(h\) can vary.

(i) Show that \(V=300\pi r-\pi r^3\).

(ii) Find the stationary value of \(V\) and determine its nature.