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Re: Re: Solve Minus Sign

  • To: mathgroup at smc.vnet.net
  • Subject: [mg106345] Re: [mg106306] Re: [mg106276] Solve Minus Sign
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sun, 10 Jan 2010 03:27:52 -0500 (EST)
  • References: <31598130.1262849769674.JavaMail.root@n11> <201001080913.EAA10198@smc.vnet.net>

Actually, in this particular case, one can do this in a simpler way:

Collect[Solve[Rr/s == Rr + x, x], Rr]

{{x -> (Rr*(1 - s))/s}}

Andrzej Kozlowski



On 8 Jan 2010, at 18:13, David Park wrote:

> A pox on those minus signs!
>
> As far as I know, this requires doing surgery on expressions. Here are some
> techniques.
>
> step1 = Solve[Rr/s == Rr + x, x][[1, 1]] // Factor 
>
> x -> -((Rr (-1 + s))/s) 
>
> The first method is to use Minus on the two different factors that contain
> the minus signs.
>
> MapAt[Minus, step1, {{2, 1}, {2, 3}}] 
>
> x -> (Rr (1 - s))/s 
>
> A second method is to Simplify and then use Simplify again on a specific
> part.
>
> Simplify[step1] 
> MapAt[Together, %, {{2, 2}}] 
>
> x -> Rr (-1 + 1/s) 
> x -> (Rr (1 - s))/s 
>
> With Presentations you can also operate on a specific subset of level parts,
> in this case the two terms in the product.
>
> Needs["Presentations`Master`"] 
>
> step1 // MapLevelParts[Expand, {2, {1, 3}}] 
>
> x -> (Rr (1 - s))/s 
>
> As far as I know, regular Mathematica doesn't have a way of operating
> together on a subset of level parts in a sum or product.
>
> Another method with Presentations is to FactorOut a -1 from s-1 and
> temporarily hold the result until the -1 can combine with the rest of the
> expression.
>
> MapAt[FactorOut[-1, HoldForm], step1, {{2, 3}}] // ReleaseHold 
>
> x -> (Rr (1 - s))/s 
>
>
> In general, the two main techniques for doing surgery on expressions are:
>
> 1) Operate on only selected portions of an expression using MapAt,
> ReplacePart, or MapLevelParts.
> 2) Shield portions of a expression using HoldForm, or CreateSubexpression in
> Presentations, so Mathematica won't split them up when doing simplifications
> on the larger expression.
>
>
> David Park
> djmpark at comcast.net
> http://home.comcast.net/~djmpark/ 
>
>
> From: Fabian [mailto:fabian.uriarte at gmail.com]
>
>
> Dear Group-
>
> How can one get rid of the minus sign in:
>
> In:
> Factor[Solve[Rr/s == Rr + x, x]]
>
> Out:
> {{x -> -((Rr (-1 + s))/s)}}
>
> We'd like Mathematica to provide answers with the minus sign
> "multiplied through" (remove Factor[] doesn't solve the problem)
>
> Thank you in advance all.
>
>
>



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