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Re: Help with findroot

  • To: mathgroup at
  • Subject: [mg9225] Re: [mg9169] Help with findroot
  • From: Hugh Walker <hwalker at>
  • Date: Fri, 24 Oct 1997 01:00:38 -0400
  • Sender: owner-wri-mathgroup at

Karl Kevala writes:

> I'm having a problem using findroot to solve an equation.  Perhaps
> someone
>  could shed some light on what's wrong.
> FindRoot[Sqrt[x/(1.2 10^ -4)]==-0.1*Tan[Sqrt[x/(1.2*10^
> -4)]],{x,0.1,0.1,2}]
> Mathematica 3. returns a value of -0.07 for x which is not anywhere
> close to correct.
>  Further, I've tried several different starting values and min/max
> limits, but
>  a negative answer is always returned.  Eventually I'de like to compile
> a list
>  of all the roots of the equation up to, say, x=1000, but I can't even
> get one
>  right now.
> Thanks,

I suggest you try the following approach. First, rescale your function
so that
                            x/1.2 10^(-4) -> z Then find the solutions
of Sqrt[z] == -0.01 Tan[Sqrt[z]]. But the solutions of this equation
are the same as the solutions of
                     Sqrt[z]Sin[Sqrt[z]] == -0.01 Cos[Sqrt[z]] This
problem has none of the difficulties encountered in the previous

Hugh Walker
Gnarly Oaks
Phone: (713) 729-3093

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