Re: Strange Solve result after previous bad input

*To*: mathgroup at smc.vnet.net*Subject*: [mg100975] Re: [mg100937] Strange Solve result after previous bad input*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Thu, 18 Jun 2009 20:50:17 -0400 (EDT)*References*: <200906181019.GAA24606@smc.vnet.net>

On 18 Jun 2009, at 19:19, 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 > In your first attempt you unintentionally defined b to be -x + y. So, in your second attempt he second equation is - x + y == - x+ y, which is a tautology. As a result your system amounts only to one equation a== x+y. The solution is simple, just Clear[b]. Andrzej Kozlowski

**References**:**Strange Solve result after previous bad input***From:*Alain Cochard <alain@geophysik.uni-muenchen.de>