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Re: Simultaneous equation solve taking ages

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114371] Re: Simultaneous equation solve taking ages
  • From: Daniel Lichtblau <danl at wolfram.com>
  • Date: Thu, 2 Dec 2010 05:40:50 -0500 (EST)

Peter Bone wrote:
> I'm trying to execute this solve. I ran it for 5 hours and it hasn't
> finished yet. Will this ever finish? Why is it taking so long?
> 
> Solve[a^2 == d^2 + f^2 - 2*d*f*x && b^2 == e^2 + f^2 - 2*e*f*y && c^2
> == d^2 + e^2 - 2*d*e*z, {d, e, f}]
> 
> I haven't used Mathematica much before so I'm not sure I've done it
> correctly. This is for a geometrical problem involving 3 cosine rule
> equations. a, b, c, x, y, z are known. Trying to find d, e, f in terms
> of the knowns.
> 
> Pete

In Mathematica 8:

In[38]:= polys = {-a^2 + d^2 + f^2 - 2*d*f*x, -b^2 + e^2 + f^2 -
     2*e*f*y, -c^2 + d^2 + e^2 - 2*d*e*z};

In[46]:= Timing[
  soln = Solve[polys == 0, {d, e, f}, Cubics -> False,
     Quartics -> False];]
Out[46]= {15.1117, Null}

In[48]:= LeafCount[soln]
Out[48]= 29163433

At this point I would raise the question of how badly you really want 
this solution?

Daniel Lichtblau
Wolfram Research


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