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MathGroup Archive 2004

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Re: Problem with Mathematica driving me nuts

  • To: mathgroup at smc.vnet.net
  • Subject: [mg46825] Re: [mg46791] Problem with Mathematica driving me nuts
  • From: Tomas Garza <tgarza01 at prodigy.net.mx>
  • Date: Tue, 9 Mar 2004 04:31:16 -0500 (EST)
  • References: <200403080910.EAA10442@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Unable to reproduce your results. I'm using Version 5.0.1 under Windows XP.
Of course, you'll never get x->2, since Mathematica uses a numerical method
(see Implementation Notes in the Help Browser: Polynomial root finding is
done based on the Jenkins-Traub algorithm). My approximation is better than
yours, anyway:

In[1]:=
FindRoot[x^2 == 4*x - 4, {x, 1}]
Out[1]=
{x -> 1.9999999701976776}

In[2]:=
FindRoot[x^2 - 4*x + 4, {x, 3}]
Out[2]=
{x -> 2.0000000298023224}

In this case, however, you may try Solve:

In[3]:=
Solve[x^2 == 4*x - 4, x]
Out[3]=
{{x -> 2}, {x -> 2}}

Tomas Garza
Mexico City
----- Original Message ----- 
From: "benwoodward.com" <bpw67deletethis at hotmail.com>
To: mathgroup at smc.vnet.net
Subject: [mg46825] [mg46791] Problem with Mathematica driving me nuts


> FindRoot[x^2 == 4x - 4, {x, 1}]
>
> Out[4]=
> {x -> 1.99902}
>
> In[15]:=
> FindRoot[x^2 - 4*x + 4, {x, 3}]
>
> Out[15]=
> {x -> 2.00098}
>
> When the root is clearly two.
> Is Mathematica using Newton's Method like a Ti-92?
> Even if so, why wont it give a more accurate answer?
> I've tried N[%,30] but it doesn't do anything.
> I'm new to Mathematica coming over from a Ti-92, so everything is
> frustrating right now.
> Thanks.
>
>



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