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Factoring

• To: mathgroup at smc.vnet.net
• Subject: [mg65069] Factoring
• From: "Matt" <anonmous69 at netscape.net>
• Date: Tue, 14 Mar 2006 05:59:48 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

Hello Mathgroup,
  I'm sure that I've overlooked something obvious, but for the past two
and a half hours, I've been trying to figure out how to use built-in
Mathematica functions to just factor the imaginary number 'I' out of
this:

4*I*r^2*Sin[2*θ] + I*r^5*Sin[5*θ]

using Factor[] gives me I*r^2*(4*Sin[2*θ] + r^3*Sin[5*θ])

using FactorTerms[] gives me I*(4*r^2*Sin[2*θ] + r^5*Sin[5*θ]) which
is what I want, but as soon as I add in a common numerical factor, it
also factors that out as well,

e.g. 4*I*r^2*Sin[2*θ] + 8*I*r^5*Sin[5*θ]

I finally just decided to divide the whole thing by 'I', and do some
reconstruction of my expression, but I have the distinct feeling I've
totally missed something very simple.  As an additional example, what
if I just wanted to extract 2*x*I out of the following:

4*x*I*Sin[t] + 28*x^3*I*Cos[t]

how would I do that?  In general, I'm looking for a function that says
"Given an expression 'expr' and another expression 'sub' that is common
to all additive terms of 'expr', give a result that is the product of
'sub' and the result of factoring 'sub' out of 'expr'".

Thanks,

Matt

• Follow-Ups:
  • Re: Factoring
    • From: Murray Eisenberg <murray@math.umass.edu>
  • Re: Factoring
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>