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Re: Beware of NSolve


Old version 2.2 does not report roots 1 and 4 by NSolve. May be later
on improvements (complex roots) have associated problems, may be
elimination of extraneous roots by plotting is a necessary check.

carlos at colorado.edu (Carlos Felippa) wrote in message news:<cfuq2n$64h$1 at smc.vnet.net>...
> 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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