Trouble with FindRoot

• To: mathgroup at smc.vnet.net
• Subject: [mg8787] Trouble with FindRoot
• From: Torbjorn Randahl <ekatra at eka.ericsson.se>
• Date: Thu, 25 Sep 1997 12:26:21 -0400
• Organization: Ericsson Components AB NL/ME
• Sender: owner-wri-mathgroup at wolfram.com

```Hi!

Why does this not work on MMA? My HP48GX solves this equation
whitin 6 seconds!

I've tried with Solve and NSolve also...

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

d=3.5

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

Out=
FindRoot::"frjc":
"Could not symbolically find the Jacobian of .
{-71+0.0170395 Re[<<1>>]}
Try giving two starting values for each variable.

r is real changing with f, the same with l
s is jw (complex)
c & d are constants

It looks like the solver beleves that there are several unknown
variables.

if I set f to (from my HP48GX)
In[55]:=
f:=714880

In[56]:=
yd

Out[56]=
71.0002

How should I have Solved this??

Torbjorn Randahl
Ericsson Components AB
Kista
Sweden

```

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