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