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Re: Solve[] doesn't

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90632] Re: [mg90619] Solve[] doesn't
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 17 Jul 2008 05:33:41 -0400 (EDT)
  • Reply-to: hanlonr at cox.net

eqn = {x - y == 1, x^2 + y^2 == 3, x^3 - y^3 == 4};

soln = Solve[eqn, {x, y}]

{{x -> (1/2)*(1 - Sqrt[5]),
     y -> (1/2)*(-1 - Sqrt[5])},
   {x -> (1/2)*(1 + Sqrt[5]),
     y -> (1/2)*(-1 + Sqrt[5])}}

Reduce[eqn, {x, y}]

(x == (1/2)*(1 - Sqrt[5]) ||
      x == (1/2)*(1 + Sqrt[5])) &&
   y == x - 1


Bob Hanlon

---- Hauke Reddmann <fc3a501 at uni-hamburg.de> wrote:

=============
This is extremely annoying.    
I have a very large (this seems to be critical) equation
system. It's partly linear, partly horror, but already
after I solve the linear part, a certain solution (which
is obviously one since substituting it in makes all equations
identical zero) can't be found anymore. The solution has some
surds (this also seems critical, the ones without are found)
but the equations are strictly polynomic.

Faked Example (the real one only PM-ed, since it's a bit
oversized):

x-y=1
x^2+y^2=3
x^3-y^3=4
and after y was eliminated, it doesn't find x. (=golden mean here)
 
Huh?
--
Hauke Reddmann <:-EX8    fc3a501 at uni-hamburg.de
    Er-a svo gott     sem gott kve=C3=B0a
    =C3=B6l alda sonum,     =C3=BEv=C3=AD a=C3=B0 f=C3=A6rra veit
    er fleira drekkur     s=C3=ADns til ge=C3=B0s gumi.




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