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

In:= LeafCount[soln]
Out= 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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