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Re: Equation solving problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg125075] Re: Equation solving problem
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Mon, 20 Feb 2012 02:52:18 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jhqmm3$fae$1@smc.vnet.net>

On 2/19/2012 1:33 PM, Juhász Péter wrote:
> I have the following problem: I would like to solve a set of 3
> equations, but Mathematica's solution involves unknown variables. My
> input was:
>
> Solve[{nc^2/Sqrt[1 - v^2/c^2] == mc^2/Sqrt[1 - b^2/c^2] + pc,
>    nv/Sqrt[1 - v^2/c^2] == (mb Cos[\[Alpha]])/Sqrt[1 - b^2/c^2] +
>      p Sin[\[Alpha]], (mb Sin[\[Alpha]])/Sqrt[1 - b^2/c^2] ==
>     p Cos[\[Alpha]]}, {v, b, p}]
>
> And the output started like:
>
> {{v ->  -(I c \[Sqrt](mc^4 nv^2 Cos[\[Alpha]]^2 -
>            2 mb mc^2 nc^2 nv Cos[\[Alpha]]^3 +
>            mb^2 nc^4 Cos[\[Alpha]]^4 - mb^2 pc^2 Cos[\[Alpha]]^4 -
>            2 mb mc^2 nc^2 nv Cos[\[Alpha]] Sin[\[Alpha]]^2 +
>            2 mb^2 nc^4 Cos[\[Alpha]]^2 Sin[\[Alpha]]^2 -
>            2 mb^2 pc^2 Cos[\[Alpha]]^2 Sin[\[Alpha]]^2 +
>            mb^2 nc^4 Sin[\[Alpha]]^4 -
>            mb^2 pc^2 Sin[\[Alpha]]^4))/(mb pc Sqrt[
>         Cos[\[Alpha]]^4 + 2 Cos[\[Alpha]]^2 Sin[\[Alpha]]^2 +
>          Sin[\[Alpha]]^4]),
>    b ->  -1/(nv pc) (\[Sqrt](-c^2 mc^4 nv^2 + c^2 nv^2 pc^2 +
>           2 c^2 mb mc^2 nc^2 nv Cos[\[Alpha]] -
>           c^2 mb^2 nc^4 Cos[\[Alpha]]^2 -
>           2 c^2 mb^2 nc^4 Sin[\[Alpha]]^2 +
>           2 c^2 mb mc^2 nc^2 nv Sin[\[Alpha]] Tan[\[Alpha]] -
>           c^2 mb^2 nc^4 Sin[\[Alpha]]^2 Tan[\[Alpha]]^2)),
>           .... .... ....
>
> And so on. It is clear, that if we look at the solution for v, it
> involves unknowns b and p. If anyone could tell me what I did wrong, I
> would highly appreciate it.
>

Mathematica treats nc as a single variable with a two latter name.  If 
you mean the product of n and c, you need to write n c (note that space 
between them) or n*c.

The solution it gives you does not include "v", but only "nv" (and other 
similar variables).


-- 
Szabolcs Horvát
Visit Mathematica.SE:  http://mathematica.stackexchange.com/



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