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Re: Strange Solve result after previous bad input

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100948] Re: [mg100937] Strange Solve result after previous bad input
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 18 Jun 2009 20:45:16 -0400 (EDT)
  • Reply-to: hanlonr at cox.net

Clear[a, b, x, y]

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

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

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

Solve[{a\[LongEqual]x+y,y-x},{x,y}]

Note in the output that in the place of the second equation there is the value (y-x). This is the result from the assignment to b. At this point b has a value

b

y-x

Just Clear b

Clear[b]

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

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


Bob Hanlon

---- 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




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