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Solve output depends on previous attempt with bad syntax

• To: mathgroup at smc.vnet.net
• Subject: [mg132501] Solve output depends on previous attempt with bad syntax
• From: Alain Cochard <Alain.Cochard at unistra.fr>
• Date: Thu, 3 Apr 2014 02:16:01 -0400 (EDT)
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First, the correct solution of a linear system of 2 equations with 2
unknowns, for reference.

    Mathematica 9.0 for Linux x86 (64-bit)
    Copyright 1988-2013 Wolfram Research, Inc.

    In[1]:= Solve[{y1==a x1 +b, y2==a x2 +b},{a,b}]

                     -y1 + y2          x2 y1 - x1 y2
    Out[1]= {{a -> -(--------), b -> -(-------------)}}
                     x1 - x2              x1 - x2

Now, starting a new Mathematica session, assume I make a mistake, using '='
instead of '==', I get an error message -- so far, so good:

    Mathematica 9.0 for Linux x86 (64-bit)
    Copyright 1988-2013 Wolfram Research, Inc.

    In[1]:= Solve[{y1=a x1 +b, y2=a x2 +b},{a,b}]

    Solve::naqs: b + a x1 && b + a x2 is not a quantified system 
    of equations and inequalities.

    Out[1]= Solve[{b + a x1, b + a x2}, {a, b}]

Realizing my mistake, I retry with the proper syntax:

    In[2]:= Solve[{y1==a x1 +b, y2==a x2 +b},{a,b}]

    Out[2]= {{}}

which is obviously not the correct result.  

Is there a rationale here, i.e., could I have anticipated this output?

Thank you,
Alain

-- 
EOST (Ã?cole et Observatoire des Sciences de la Terre) 
IPG (Institut de Physique du Globe) | alain.cochard at unistra.fr
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• Follow-Ups:
  • Re: Solve output depends on previous attempt with bad syntax
    • From: Bob Hanlon <hanlonr357@gmail.com>