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How do I create a parametric expression?
 To: mathgroup at smc.vnet.net
 Subject: [mg68537] How do I create a parametric expression?
 From: axlq at spamcop.net (axlq)
 Date: Wed, 9 Aug 2006 04:19:05 0400 (EDT)
 Sender: ownerwrimathgroup at wolfram.com
I'm trying to figure out how to simplify a large expression so that it's
expressed in terms of a subexpression that's factored into the larger
one.
My expression looks like this:
((1 + 2*n)*((a^4*k^2 + a^2*(1 + k^2*(q  z)^2) + 2*(q  z)^2)
*Cos[k*Sqrt[a^2 + (q  z)^2]]  k*(a^2  2*(q  z)^2)
*Sqrt[a^2 + (q  z)^2]*Sin[k*Sqrt[a^2 + (q  z)^2]])
*Sin[((1 + 2*n)*Pi*z)/L])/(8*Pi*w*(a^2 + (q  z)^2)^(5/2))
Now, I *know* there are places in there were Sqrt[a^2+(qz)^2] occurs,
either by itself or raised to various powers. If I want to define
R:=Sqrt[a^2+(qz)^2]
...then how can I make Mathematica restate my expression in terms
of R? The ReplaceRepated[] function doesn't seem to do the job.
I need to do this because I am translating the expressions into
Visual Basic code for an Excel application, and it would be nice to
find groupings of terms repeated throughout the expression that I
need to calculate only once.
Alex
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