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Re: smart change of variables?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg87745] Re: smart change of variables?
  • From: "David Park" <djmpark at comcast.net>
  • Date: Wed, 16 Apr 2008 05:07:08 -0400 (EDT)
  • References: <fu21bk$rb6$1@smc.vnet.net>

(eqns = {expr == (p1 + 2 p2) (k1 + 2 k2), p1 + p2 == k1 + k2,
    s == (p1 + p2)^2, t == (p1 - k1)^2, u == (p1 - k2)^2}) // Column

Solve[eqns, expr, {k1, k2, p1, p2}]
{{expr -> 1/4 (9 s - t - 6 Sqrt[s] Sqrt[u] + u)},
  {expr -> 1/4 (9 s - t + 6 Sqrt[s] Sqrt[u] + u)}}

-- 
David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/


"Barrow" <GRseminar at gmail.com> wrote in message 
news:fu21bk$rb6$1 at smc.vnet.net...
> Dear all,
>
> I have a arithmetic problem.
> I have an expression expr = (p1 + 2*p2)*(k1 + 2*k2)
> where p1 + p2 = k1 + k2
> I wanna make the following change of variables,
> s = (p1 + p2)^2
> t = (p1 - k1)^2
> u = (p1 - k2)^2
>
> Is it possible to tell Mathematica to express expr
> in terms of s, t, and u automatically?
>
> Thanks so much.
> any ideas would be appreciated.
> Sincerely Barrow
> 



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