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Re: Simple Concept I can't figure out! Making rules?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg38080] Re: Simple Concept I can't figure out! Making rules?
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Tue, 3 Dec 2002 04:28:51 -0500 (EST)
  • References: <as5pgp$n96$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Stan,

We have

    fullsolution = Solve[{x + y == 2, x^2 + y^2 == 10}, {x, y}]

        {{x -> -1, y -> 3}, {x -> 3, y -> -1}}

The full solution contains two answers {x->-1,y->3} and {x->3,y->-1}

This lets us state the full solution in a way that relates to the variables
and also lets us do things like

    a x + b y /. solution

    {-a + 3 b, 3 a - b}

Which gives the results of substituting using each of the two answers.


With your example

    fullsolution = N[Solve[2 x - 1 == 0, x]]

        {{x -> 0.5}}

We can get

     answer = x /. fullsolution[[1]]

        0.5

    answer

        0.5
--
Allan

---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565


"Stan" <snarten at runbox.com> wrote in message
news:as5pgp$n96$1 at smc.vnet.net...
> Even though it's frowned upon by the engineering college, i use
> mathematica a lot of the stuff that would take pages of code in
> another system. I have most basic functions and operations down, but I am
> lacking one basic thing I need to do to be able to program more
> problems successfully:
>
> here is my problem:
>
> Lets say you solve an equation using "Solve"
>
> Solve[2x - 1 == 0, x] // N
>
> {{x -> 0.5}}
>
> How would I then use the result of "Solve" (0.5) and assign it to a
> new variable, like "answer"  for further calculations?
>
> I think it has something to do with /. and -> rules of some sort, but
> I can't get this simple concept to work!
>
> Thanks for the help.
>
> -Stan
>




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