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Topic: How to get the Real and Imaginary part of an expression
Replies: 10   Last Post: Jan 5, 2013 2:18 AM

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 Murray Eisenberg Posts: 2,105 Registered: 12/6/04
Re: How to get the Real and Imaginary part of an expression
Posted: Jan 5, 2013 2:18 AM

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:

ComplexExpand[Re[(w)/(s^2 + 2*z*w*s + w^2) /. s -> x + I y]]
w^3/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2) +
(w*x^2)/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2) -
(w*y^2)/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2) +
(2*w^2*x*z)/((w^2 + x^2 - y^2 + 2*w*x*z)^2 + (2*x*y + 2*w*y*z)^2)

And similarly for Im.

(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 <emammendes@gmail.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
>
>

---
Murray Eisenberg murray@math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower phone 413 549-1020 (H)
University of Massachusetts 413 545-2838 (W)
710 North Pleasant Street fax 413 545-1801
Amherst, MA 01003-9305

Date Subject Author
1/5/13 R Martinez
1/5/13 James Stein
1/5/13 Bob Hanlon
1/5/13 Murray Eisenberg
1/5/13 Murray Eisenberg
1/5/13 emammendes@gmail.com
1/5/13 Murray Eisenberg