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Re: Strange Solve result after previous bad input

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100946] Re: Strange Solve result after previous bad input
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Thu, 18 Jun 2009 20:44:55 -0400 (EDT)
  • Organization: Uni Leipzig
  • References: <h1d4ao$o0q$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de

Hi,

yes this is normal. Because the false
input " b=-x + y " set b to y-x, in every
expression that contain b it will be replaced by
y-x  and the second input of
Solve[{a==x + y, b==-x + y},{x,y}]
is evaluated to
Solve[{a==x + y, -x+y==-x + y},{x,y}]
and future to
Solve[{a==x + y, True},{x,y}]

and only one equation is left to determine x and y.

Regards
   Jens

Alain Cochard wrote:
> This is the proper solution to the correctly input system:
> 
>      Mathematica 6.0 for Linux Itanium (64-bit)
>      Copyright 1988-2007 Wolfram Research, Inc.
> 
>      In[1]:= Solve[{a==x + y, b==-x + y},{x,y}]
> 
> 		    a - b       a + b
>      Out[1]= {{x -> -----, y -> -----}}
> 		      2           2
> 
> Here, there is '=' instead of '==' in the 2nd eq:
> 
>      In[2]:= Solve[{a==x + y, b=-x + y},{x,y}]
> 
>      Solve::eqf: -x + y is not a well-formed equation.
> 
>      Out[2]= Solve[{a == x + y, -x + y}, {x, y}]
> 
> and then, subsequent properly input systems will still fail:
> 
>      In[3]:= Solve[{a==x + y, b==-x + y},{x,y}]
> 
>      Solve::svars: Equations may not give solutions for all "solve" variables.
> 
>      Out[3]= {{x -> a - y}}
> 
> 
> Is this normal?  
> 
> If so, is there a cure (some "resetting" procedure), apart from
> quiting Mathematica and entering back?
> 
> Also, is there a way of testing that a previous bad input is indeed
> the cause of that strange output? (Because if it happened a long time
> ago in the Mathematica history, one might no longer remember.)
> 
> Thanks
> AC
> 


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