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MathGroup Archive 2004

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Re: Beware of NSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50181] Re: [mg50165] Beware of NSolve
  • From: Murray Eisenberg <murray at math.umass.edu>
  • Date: Thu, 19 Aug 2004 06:28:12 -0400 (EDT)
  • Organization: Mathematics & Statistics, Univ. of Mass./Amherst
  • References: <200408180520.BAA06016@smc.vnet.net>
  • Reply-to: murray at math.umass.edu
  • Sender: owner-wri-mathgroup at wolfram.com

But:

    NSolve[f == 0, x, WorkingPrecision -> 32]
{{x -> -0.014126116704662366388244906316568},
{x -> 0.0028165928951385568644353825070445}}


Carlos Felippa wrote:
> Run v. 4.2 on Mac:
> 
> f=5/432 - 11/(27*Sqrt[70]*Sqrt[19 - 1890*x]) + x/(2*Sqrt[38/35 - 108*x]);
> 
>  Solve[f==0,x]  returns 2 real roots:
> 
> {{x -> (-171 - 25*Sqrt[105])/30240}, {x -> (-171 + 25*Sqrt[105])/30240}}
> 
> NSolve[f==0,x]  returns 4 real roots:
> 
> {{x -> -0.10481082961146104}, {x -> -0.014126116704662378}, 
>  {x -> 0.002816592895138556}, {x -> 0.0003796126802330315}}
> 
> Roots 1 and 4 are incorrect.  (Just plot f)
> 
> Had a similar problem with a quartic 3 months ago.  This is a
> simpler example.
> 
> 

-- 
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


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