Since there seems to be some typo or else some spurious control code ("=882") in the numerator of your fraction, for purposes of explanation I'll change the numerator just to w.
In general, the way to extract the real and imaginary parts of a complex number is to use ComplexExpand along with, of course, Re and Im. Here, though, you have both real and complex variables, so I think you'll need to express the complex s in the form x + I y. Then applying ComplexExpand will treat all the variables w, z, x, and y as real:
(I've shown the results in one-dimensional InputForm for purposes of this plain-text e-mail.)
On Jan 2, 2013, at 9:16 PM, Eduardo M. A. M. Mendes <email@example.com> wrote:
> Hello > > I need to extract the real and imaginary part of the following expression > > (w=882)/(s^2+2*z*w*s+w^2) > > where w and z are positive constants. s is a complex variable. > > Applying Re and Im to the expression does not do much. By hand, one can easily find them. > > What am I missing? > > many thanks > > Ed > >
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