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MathGroup Archive 2007

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Using FindRoot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83429] Using FindRoot
  • From: tdude <stiletto at bigfoot.com>
  • Date: Tue, 20 Nov 2007 03:51:58 -0500 (EST)

I am trying to find the root of the following equation
FindRoot[.15*[Pi]*(R^2)==(2*R^2*ArcCos[45.9/(2*R)])-(.5*45.9*Sqrt[4*R^2-45.9^2]),{R,.1}, MaxIterations->100000]

When I try this with two other systems, both give me an answer of 30.8773686.

However, with Mathematica, the answer I get appears to be a complex root, along with this message:
FindRoot::lstol: The line search decreased the step size to within tolerance specified by AccuracyGoal and PrecisionGoal but was unable to find a sufficient decrease in the merit function.  You may need more than MachinePrecision digits of working precision to meet these tolerances.
{R -> 20.1137- 4.87774*10^-14 \[ImaginaryI]}

Why would the answer be a complex root when using Mathematica, yet real when using the other two packages?
Thanks Tony.


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