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