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
>