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. > > >

**References**:**Re: Solve Minus Sign***From:*"David Park" <djmpark@comcast.net>