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Solve[] handles the same system differently

  • To: mathgroup at smc.vnet.net
  • Subject: [mg40841] Solve[] handles the same system differently
  • From: Joe Gwinn <joegwinn at attbi.com>
  • Date: Mon, 21 Apr 2003 06:59:15 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

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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