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