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Re: Strange Solve result after previous bad input

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100969] Re: Strange Solve result after previous bad input
  • From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
  • Date: Thu, 18 Jun 2009 20:49:11 -0400 (EDT)
  • References: <h1d4ao$o0q$1@smc.vnet.net>

Well, you have given b the value of -x+y in [2] so in [3] the second
equation reduces to True.

The solution: clear b either by using ClearAll[b] or b=.


Cheers -- Sjoerd

On Jun 18, 12:19 pm, Alain Cochard <al... at geophysik.uni-muenchen.de>
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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