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Re: Solve Minus Sign

  • To: mathgroup at smc.vnet.net
  • Subject: [mg106306] Re: [mg106276] Solve Minus Sign
  • From: "David Park" <djmpark at comcast.net>
  • Date: Fri, 8 Jan 2010 04:13:26 -0500 (EST)
  • References: <31598130.1262849769674.JavaMail.root@n11>

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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