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Re: FindRoot

  • To: mathgroup at
  • Subject: [mg8822] Re: [mg8793] FindRoot
  • From: Allan Hayes <hay at>
  • Date: Fri, 26 Sep 1997 00:33:45 -0400
  • Sender: owner-wri-mathgroup at

Torbjorn Randahl <ekatra at>
[mg8793] FindRoot
writes as below the line *****

Changing the input slightly (giving two initial values as suggested  
in the message that you got)I get (on a 33 MHz NeXT slab)

FindRoot[yd == 71, {f, 300000,300001}]//Timing

{0.124536 Second,{f->714877.}}

Allan Hayes
hay at
voice:+44 (0)116 2714198
fax: +44 (0)116 2718642
Leicester,  UK


Mma can't find f (the frequency) for a value of yd, my HP48GX find it
within 6 seconds!

yd is the Real part of the root of (r+sl)sc

r changes with f, the same with l. c and d are constants

s is jw (complex)

This is the Mma run:
yd := (20*d*Re[Sqrt[(r + l*s)*s*c]])/Log[10]

s := f*I*2*Pi

r := (((f*2*Pi)^1.198*2.18)/10^9 + 0.276)*1000

l := (587 - (f*92.37)/10^6)/10^6

c = 50/10^9


FindRoot[yd == 71, {f, 300000, 30000, 1.1*10^6}]

    "Could not symbolically find the Jacobian of
    {-71+0.0170395 Re[<<1>>]}. Try giving two starting
    values for each variable."
(I've tried Solve,Nsolve also)

If I then set f to 714880 (from my HP48GX)



So if I put in the ansewer Mma gives the right result of yd!
How should I Solve this?

best regards
Tobbe Randahl
Ericsson Components AB
email: ekatra at

Torbjorn Randahl

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