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Re: Expression manipulation

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  • Subject: [mg88718] Re: Expression manipulation
  • From: Szabolcs Horvát <szhorvat at>
  • Date: Wed, 14 May 2008 06:59:52 -0400 (EDT)
  • Organization: University of Bergen
  • References: <g0edhe$9eq$>

David wrote:
> When trying to simplify an expression by hand, one carries out various
> kinds of steps:
> 1. Replace a subexpression that occurs repeatedly by a single symbol.
> 2. Multiply numerator and denominator of some subexpression by the
> same factor.
> 3. Cancel particular factors in numerator and denominator of some
> subexpression.
> 4. Gather together two subexpressions that were added together, and
> rewrite with a common denominator.
> 5. Remove common factors.
> etc. etc. etc.
> Using Part, one can of course access any particular subexpression. But
> this is time-consuming and clumsy. I find that I need trial and error
> to access the correct subexpression. Once I've accessed it, I often
> have difficulty in persuading Mathematica to perform the desired
> operation. And then I have trouble putting the subexpression back into
> place. It's something like 20 times slower than working with pencil
> and paper. HOWEVER pencil and paper calculations are more prone to
> stupid arithmetic errors, particularly if the computation is a long
> one.
> I have been unable to find a convenient way of doing this in
> Mathematica. I use version 5.2, but because of my University's site
> license, I have access to more recent versions. Would it help to
> change?
> Can anyone point me to a tutorial where experts use Mathematica to do
> a typical pencil and paper computation?

I prefer pen and paper to Mathematica when manipulating simple 
expressions.  Mathematica is useful when working with large expressions, 
e.g. high-order polynomials or rational functions, etc.

Don't use Part during interactive work.  Instead take a look at 
ReplaceAll, Together, Expand, ExpandAll, Cancel, etc.  Or just copy and 
paste when you need to extract a particular subexpression.

> Thanks a lot. Please copy replies to my personal email address as I
> don't look at the newsgroup very often.

This just tells us that you don't really care about the reply ... so 
fewer people will bother to answer ...

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