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Re: Solve[] doesn't

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90626] Re: [mg90619] Solve[] doesn't
  • From: Curtis Osterhoudt <cfo at lanl.gov>
  • Date: Thu, 17 Jul 2008 05:32:34 -0400 (EDT)
  • Organization: LANL
  • References: <200807161029.GAA08789@smc.vnet.net>
  • Reply-to: cfo at lanl.gov

  Is your syntax correct? Especially your equal signs are assignments, rather 
than tests for equality. When those are changed, at least in v. 6.0.3 for 
linux, things work fine: 

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

Solve[eqns, {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])}}

   OR

Eliminate[eqns, y]
-x + x^2 == 1

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

On Wednesday 16 July 2008 04:29:42 Hauke Reddmann 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?



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