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MathGroup Archive 1998

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Re: I don't understand in Mathematic



At 4:53 PM -0500 1/20/98, alok. menghrajani wrote:
>Hi,
>
>I'm 16 and I can't understand how I (why me?) found this bug in
>mathematica. It's such a silly thing:
>
>Solve [x/x+x=1, x] returns x=0 but if x=0 then x/x has no sence!
>
>Bye, ALok
>
>(Pls reply to my email: robot@writeme.com)

Well, this isn't really a bug, if you will agree that something that is
intended isn't a bug.
When performing symbolic manipulations Mathematica assumes that
variables are "generic" and does not pay attention to "special cases",
like x=o in your example. In fact x/x always returns 1. So when doing
Solve[x/x+x==1,x] Mathematica first replaces x/x by 1 and then solves
the equation 1+ x== x. Mathematica knows very well that 0/0 is not
really 1, but its just a matter of the order in which evaluation is
performed. Compare the results of
x/x/.{x->0}
and
Unevaluated[x/x]/.{x->0}

In the second case Mathematica notices correctly that 0/0 has no
meaning. This automatic replcement of x/x by 1 is not a bug, but is
essentially inevitable in a symbolic algebra program (I think!).
Remember that Mathematica is a computer program, not a person! If
Mathematica always had to check that expressions are non zero before it
cancelled them  symbolic computations would become impossibly complex.

Of course, it would be possible to make Mathematica check its answers by
substituting them into uneveluated equations and then evaluating them,
as in the above example, but it would be of minor importance and you
can easily do it yourself.

There are also various ways of using Solve, which avoid such problems.
Fo example  system of equations :


Solve[{x/u+x==1,x==u},x]
{}

which is equivalent to your equation, gives the correct answer .




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