       Re: Finding the real part of a symbolic complex expression

• To: mathgroup at smc.vnet.net
• Subject: [mg126582] Re: Finding the real part of a symbolic complex expression
• From: "Nasser M. Abbasi" <nma at 12000.org>
• Date: Mon, 21 May 2012 05:56:32 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <jpa3ea\$468\$1@smc.vnet.net>
• Reply-to: nma at 12000.org

```On 5/20/2012 1:36 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?
>

--------------------
expr=(a+I b)(c+I d);
ComplexExpand[Re[expr]]
----------------------
Out= a c-b d

-----------------------------
Assuming[{Element[{a,b,c,d},Reals]},Simplify[Re[Expand[expr]]]]
-----------------------------
Out= a c-b d

--Nasser

```

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