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RE : Beware of NSolve
*To*: mathgroup at smc.vnet.net
*Subject*: [mg50168] RE : [mg50165] Beware of NSolve
*From*: "Florian Jaccard" <florian.jaccard at eiaj.ch>
*Date*: Wed, 18 Aug 2004 04:34:03 -0400 (EDT)
*Sender*: owner-wri-mathgroup at wolfram.com
Very interesting... thank you to have taken time to show this.
It is a good example to show some dangers...
In fact, the Help browser says the following :
?NSolve
"NSolve[lhs==rhs, var] gives a list of numerical approximations to the roots
of a polynomial equation...."
Your equation is not polynomial, so NSolve is not really the best way to
solve it.
As the equation is not complicated, Solve should manage...
And it does :
N[Solve[f == 0, x]]
{{x -> -0.014126116704662368},
{x -> 0.0028165928951385567}}
Greetings
F.Jaccard
-----Message d'origine-----
De : Carlos Felippa [mailto:carlos at colorado.edu]
Envoyé : mercredi, 18. août 2004 07:20
À : mathgroup at smc.vnet.net
Objet : [mg50165] Beware of NSolve
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.
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