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MathGroup Archive 2006

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Re: Why is the negative root?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69506] Re: Why is the negative root?
  • From: dh <dh at metrohm.ch>
  • Date: Thu, 14 Sep 2006 06:56:14 -0400 (EDT)
  • References: <ee8h8v$j48$1@smc.vnet.net>

Hi Peter,
what you do is just  fine, ToRadical is the culprit, that somehow mixes 
the various roots. Wolfram sholuld take note.
You can see this by:
Reduce[{z^3 - z^2 - b z - 1 == 0, b > 0, z > 0}, z] /. b -> 3.
will give the expected root.

Daniel

()
p-valko at tamu.edu wrote:
> (Reduce[{z^3-z^2- b z-1==0, b>0, z>0}, z] //ToRadicals)/.b->3.//Chop
> 
> gives a negativ z:
> 
> z == - 0.414214
> 
> What am I doing wrong?
>  
> Regards
> Peter
> 


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