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Topic: Solve output depends on previous attempt with bad syntax
Replies: 1   Last Post: Apr 4, 2014 3:58 AM

 David Park Posts: 1,560 Registered: 5/19/07
Re: Solve output depends on previous attempt with bad syntax
Posted: Apr 4, 2014 3:58 AM

Clear[ y1, y2]

David Park
djmpark@comcast.net
http://home.comcast.net/~djmpark/index.html

From: Alain Cochard [mailto:Alain.Cochard@unistra.fr]

First, the correct solution of a linear system of 2 equations with 2 unknowns, for reference.

Mathematica 9.0 for Linux x86 (64-bit)

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)

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

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