Re: Re: Solve Minus Sign

*To*: mathgroup at smc.vnet.net*Subject*: [mg106354] Re: [mg106306] Re: [mg106276] Solve Minus Sign*From*: Fabian Marcel <fabian.uriarte at gmail.com>*Date*: Sun, 10 Jan 2010 03:29:31 -0500 (EST)*References*: <31598130.1262849769674.JavaMail.root@n11>

Thank you Andrzej, that was a nice little trick. On Fri, Jan 8, 2010 at 6:26 AM, Andrzej Kozlowski <akoz at mimuw.edu.pl> wrote: > 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. >> >> >> > >