Re: Finding the real part of a symbolic complex expression

*To*: mathgroup at smc.vnet.net*Subject*: [mg126589] Re: Finding the real part of a symbolic complex expression*From*: Murray Eisenberg <murray at math.umass.edu>*Date*: Mon, 21 May 2012 05:58:58 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <201205200635.CAA04268@smc.vnet.net>*Reply-to*: murray at math.umass.edu

As you discovered, the way to handle this is with ComplexExpand: ComplexExpand[Re[(a + I b) (c + I d)]] a c - b d Clearly the function Re does not recognize such an Assuming expression. What one might expect is that the following would work: Simplify[Re[(a + I b) (c + I d)], Element[{a, b, c, d}, Reals]] Alas, it simply doesn't. However, the following does: Simplify[Re[Expand[(a + I b) (c + I d)]], Element[{a, b, c, d}, Reals]] On 5/20/12 2:35 AM, Jacare Omoplata wrote: > I wanted to find the real part of (a + I b)(c + I d) , assuming a,b,c and d are real, "I" being Sqrt[-1]. > > So I tried, > > Re[(a + I b) (c + I d)] /. Assuming -> Element[{a, b, c, d}, Reals] > > Nothing happens. What I get for output is, > > Re[(a+I b) (c+I d)] > > I found out that I can use the function "ComplexExpand" to expand the expression assuming a,b,c and d to be real. But I'm curious to know if there a way to make Mathematica use "Re" to find the real part? > -- Murray Eisenberg murray at math.umass.edu Mathematics & Statistics Dept. Lederle Graduate Research Tower phone 413 549-1020 (H) University of Massachusetts 413 545-2859 (W) 710 North Pleasant Street fax 413 545-1801 Amherst, MA 01003-9305

**References**:**Finding the real part of a symbolic complex expression***From:*Jacare Omoplata <walkeystalkey@gmail.com>