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MathGroup Archive 2000

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RE: A simple problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25820] RE: [mg25786] A simple problem
  • From: "David Park" <djmp at earthlink.net>
  • Date: Sat, 28 Oct 2000 01:41:12 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Paul,

This is one of the irritating features of Mathematica. The Version 4 package
ExpressionManipulation, which Ted Ersek helped we write, has a routine
called RootsTogether.

?RootsTogether

"RootsTogether[expr] will put factors involving the same kind of root \
together under one root sign. Like PowerExpand it is not always a
permissible \
operation. Mathematica automatically removes numeric factors from root \
expressions."

tsol = Solve[1/2*a*t^2 == p, t]
RootsTogether[%]

{{t -> -((Sqrt[2]*Sqrt[p])/Sqrt[a])},
  {t -> (Sqrt[2]*Sqrt[p])/Sqrt[a]}}
{{t -> -Sqrt[2]*Sqrt[p/a]}, {t -> Sqrt[2]*Sqrt[p/a]}}

I do not know how to get the 2 into the square root into the same root since
Mathematica automatically removes numeric factors. The package is available
at my web site below.

With nospam mail addresses you may only get a very delayed response because
nobody can send you a direct reply.

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/



> From: Paul Lutus [mailto:nospam at nosite.com]
To: mathgroup at smc.vnet.net

> I have a very simple notation problem I cannot solve (because of
> my limited
> knowledge of Mathematica).
>
> Example -- an equation from physics:
>
> f := a * t^2 / 2 - p == 0
>
> Now I want to create the different forms of this equation, such as:
>
> Solve[f,t]
>
> which produces:
>
> sqrt(2) * sqrt(p) / sqrt(a)
>
> Okay, the correct general solution. What I really wanted was the limited
> solution ( sqrt(2p/a) ). I know I need to say "assume all the
> variables are
> >= 0", but I can't figure out how to express this.
>
> I've tried applying a number of different constraints without success.
>
> Hey -- I warned you. Simple problem. :)
>
> --
>
> Paul Lutus
> www.arachnoid.com
>



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