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Re: Solve[] handles the same system differently

  • To: mathgroup at smc.vnet.net
  • Subject: [mg40873] Re: Solve[] handles the same system differently
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Tue, 22 Apr 2003 06:49:35 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <b80k0i$m3f$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

if you introduce the new equation (and a new variable)
Mathematica get two equations for the same variable and
it can not resolve it. Try

Solve[{Rx == 3300, VinX1 == 0, VgsX1 == Rx*IdsX1, 
    IdsX1 == 0.001(1 - (VinX1 - VgsX1)/(-1.5))^2}, {IdsX1, VgsX1}]

Regards
  Jens

Joe Gwinn wrote:
> 
> I am running Mathematica 4.0.1.0 under MacOS 9.1, and have encountered a
> little mystery:
> 
> The following works OK:
> 
> In[15]:=
> Solve[{Rx == 3300, VinX1 == 0,
>     IdsX1 == 0.001(1 - (VinX1 - Rx*IdsX1)/(-1.5))^2}, {IdsX1}]
> 
> Yielding:
> 
> Out[15]=
> {{IdsX1 -> 0.000234453}, {IdsX1 -> 0.00088125}}
> 
> When I rearrange the equation system a bit, it fails to find a solution:
> 
> In[16]:=
> Solve[{Rx == 3300, VinX1 == 0, VgsX1 == Rx*IdsX1,
>     IdsX1 == 0.001(1 - (VinX1 - VgsX1)/(-1.5))^2}, {IdsX1}]
> 
> Out[16]=
> {}
> 
> What's going on?  Why does a little algebra (the creation of the
> variable VgsX1) cause a problem?  In[16] was generated by copying In[15]
> and doing some editing.  No error messages were generated by either.
> 
> Thanks,
> 
> Joe Gwinn


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