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Re: how do you prevent numerator expansion when using 'Together'?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122316] Re: how do you prevent numerator expansion when using 'Together'?
  • From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
  • Date: Tue, 25 Oct 2011 06:15:55 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Hi,
You also can try the following. Assume that you have your complex expression instead of this

expr = 1/(d*g)*(a*g + b*g + a*e + b*e + a*f + b*f)


(a e + b e + a f + b f + a g + b g)/(d g)

simple one.  The third part of it is the numerator:

expr[[3]]

a e + b e + a f + b f + a g + b g

This can be also seen by the TreeForm. Then let us factor it:

MapAt[Factor, expr, {3}]

((a + b) (e + f + g))/(d g)

I hope this helps.
Best, Alexei




DrMajorBob wrote:
> Factor[(a + b)/d + ((a + b)*(e + f))/(d*g)]
>
> ((a + b) (e + f + g))/(d g)
>
> Together[(a + b)/d + ((a + b)*(e + f))/(d*g)]
>
> ((a + b) (e + f + g))/(d g)
>
>
Yes, this simplified example does this on my system too, but the actual
expression I have behaves the way I described.  Your counter-example
indicates there must be something in my actual expression that causes
the behavior.  I'll experiment to see what that might be.  Thanks for
pointing this out.

- Dushan

> On Sat, 22 Oct 2011 05:07:37 -0500, Dushan Mitrovich <dushanm at nnips.net>
> wrote:
>
>> I'm using Mathematica 8.  When using 'Together' on an expression the
>> numerator
>> is always expanded, complicating the appearance of the final result:
>>
>>      In:  Together[ (a+b)/d + ((a+b)*(e+f))/(d*g) ]
>>      Out: 1/(d*g) * (a*g + b*g + a*e + b*e + a*f + b*f)
>>
>> Instead, I'd much prefer the simpler form
>>           1/(d*g) * (a+b)*(g+e+f)
>>
>> What do I need to do to get the latter form?  Thanks.
>>
>> - Dushan
>>    [ reverse the middle word of address to reply ]
>>
>
>

Alexei BOULBITCH, Dr., habil.
IEE S.A.
ZAE Weiergewan,
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L-5326 Contern, LUXEMBOURG

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