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Re: Absolute value

  • To: mathgroup at smc.vnet.net
  • Subject: [mg110614] Re: Absolute value
  • From: Peter Pein <petsie at dordos.net>
  • Date: Mon, 28 Jun 2010 02:29:31 -0400 (EDT)
  • References: <i073n4$kk1$1@smc.vnet.net>

Am Sun, 27 Jun 2010 08:56:04 +0000 (UTC)
schrieb Marco Masi <marco.masi at ymail.com>:

> I would like to calculate the absolute value of complex quantities.
> For example Abs[Exp[I phi1]+Exp[I*phi2]], which sould give 2
> (1+cos(phi1-phi2)). However it does not work. I tried to use real
> numbers as assumtion, but it always answers "Abs[Exp[I
> phi1]+Exp[I*phi2]]". What am I doing wrong?
> 
> Regards, Mark.
> 

Hi Mark,

1.) you forgot the Sqrt around 2(1+cos(phi1-phi2)) in your posting.
2.) you did not use ComplexExpand:

ComplexExpand[Abs[Exp[I phi1] + Exp[I phi2]]] // TrigFactor will return
Sqrt[2] Sqrt[1 + Cos[phi1 - phi2]]

Peter



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