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Re: Strange Solve result after previous bad input

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100965] Re: [mg100937] Strange Solve result after previous bad input
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Thu, 18 Jun 2009 20:48:27 -0400 (EDT)
  • References: <200906181019.GAA24606@smc.vnet.net>
  • Reply-to: drmajorbob at bigfoot.com

Clear[b] or b=. will correct your mistake (using = when you needed ==).

Solve[{a==x+y,b=-x+y},{x,y}]

Solve::eqf: -x+y is not a well-formed equation. >>
Solve[{a==x+y,-x+y},{x,y}]

Clear[b]
Solve[{a==x+y,b==-x+y},{x,y}]

{{x->(a-b)/2,y->(a+b)/2}}

or

Solve[{a==x+y,b=-x+y},{x,y}]

Solve::eqf: -x+y is not a well-formed equation. >>
Solve[{a==x+y,-x+y},{x,y}]

b=.
Solve[{a==x+y,b==-x+y},{x,y}]

{{x->(a-b)/2,y->(a+b)/2}}

Bobby

On Thu, 18 Jun 2009 05:19:12 -0500, Alain Cochard  
<alain 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
>



-- 
DrMajorBob at bigfoot.com


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