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MathGroup Archive 2004

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newbie alert: FindRoot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49397] newbie alert: FindRoot
  • From: schaa at geo.uni-koeln.de (Ralf Schaa)
  • Date: Sat, 17 Jul 2004 06:39:23 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Folks,

I am programming in IDL and I have this set of nonlinear equations,
which can be solved numerically in IDL with Broyden's method ... so
far so good.
Now, when i slightly vary the initial guess the solutions vary hugely.

Now the question for the community:
I'd like to check my programming code with mathematica using the
FindRoot function like this:

Vt := 49.833666;
Vr := 49.837507;
Pt := 372331487.64643341;
Pr := 5406.0310;
Psi_t := 1.4213396;
Psi_r := 2.0724097;
Fdop := 363893.42;

FindRoot[
{Vt*Cos[Theta_t] - Vr*Cos[Theta_r] - Fdop == 0, 
Pt*Sin[Theta_t - Psi_t] + a == 0, 
Pr*Sin[Theta_r - Psi_r] + a == 0}, 
{Theta_t, 0}, {Theta_r, 0}, {a, 100}];

But all I get is something like this:
 ... "is not a length 3 list of numbers at {Theta_t,Theta_r,a} =
{0.`,0.`,100.`}."

So how has this to be done correctly ?

Thanks in advance!
-Ralf


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