For Problems 29–31: In an Argand plane \(z_1, z_2\), and \(z_3\) are, respectively, the vertices of an isosceles triangle \(ABC\) with \(AC = BC\) and \(\angle CAB = \theta\). If \(z_4\) is incenter of triangle, then the value of \(AB \times AC/(IA)^2\) is