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MathGroup Archive 1997

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Re: N and NSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7335] Re: [mg7300] N and NSolve
  • From: Allan Hayes <hay at haystack.demon.co.uk>
  • Date: Tue, 27 May 1997 22:27:10 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

23 May 1997
C L C Fu <es2136 at eng.warwick.ac.uk>
[mg7300] N and NSolve

>I sued NSolve to solve a  qubic equations. It gave me three real  
>numerical solution.  However, when I used Solve to solve the same  
>solution symbolicly, it gave me three roots in symbolic form and  
>when I used N to solve the roots numerically, it gave me three  
>solution in complex forms.
>The  real parts are the same in both cases.
>
>The solutions are  in the first case:
>
>{{Cp -> -8.104589570312425*^-8}, {Cp -> 2.522495922927543*^-8},
>  {Cp -> 4.500002377739561*^-8}}
>
> The second case:
>
> {{4.500002377739564*^-8 + 1.52201232709757*^-22*I},
>  {-8.104589570312425*^-8 + 2.977850205190899*^-23*I},
>  {2.522495922927541*^-8 - 1.720535674110297*^-22*I}}
>
>Why is that?


Col:
Ths difference comes from the details of the numerical evaluation -  
in the second case the very small imaginary parts are accidental  
and are not discarded.
You can remove them :

Chop[{{4.500002377739564*^-8 + 1.52201232709757*^-22*I},
  {-8.104589570312425*^-8 + 2.977850205190899*^-23*I},
  {2.522495922927541*^-8 - 1.720535674110297*^-22*I}}]


	{{4.500002377739564*^-8}, {-8.104589570312425*^-8},
  	{2.522495922927541*^-8}}

Allan Hayes
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk/
voice:+44 (0)116 2714198
fax: +44 (0)116 2718642
12 Copse Close, Leicester, LE2 4FB, UK




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