Simplify complex equation

*To*: mathgroup at smc.vnet.net*Subject*: [mg114406] Simplify complex equation*From*: "mailcwc at gmail.com" <mailcwc at gmail.com>*Date*: Fri, 3 Dec 2010 05:22:08 -0500 (EST)

I tried the following code which attend to extract the real and the imaginary part of q4. z, s, f, s2 are all real numbers. Why I get imaginary part in the first answer and zero in the 2nd answer? q1 = Iz; q2 = q1 + s; q3 = (q2 f)/(f - q2); q4 = q3 + s2; ComplexExpand[Re[q4]] ComplexExpand[Im[q4]] Out = s2 + (f s)/(f - s - Iz) + (f Iz)/(f - s - Iz) Out = 0