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Re: FindRoot


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

This is because you asked your calculator to do a different calculation
than you asked Mathematica to do.  The warning message in the following
calculation reflects the fact that the input is not a differentiable
function.

In[1]:= yd := (20*d*Re[Sqrt[(r + l*s)*s*c]])/Log[10]

In[2]:= s := f*I*2*Pi

In[3]:= r := (((f*2*Pi)^1.198*2.18)/10^9 + 0.276)*1000

In[4]:= l := (587 - (f*92.37)/10^6)/10^6

In[5]:= c = 50/10^9;

In[6]:= d=3.5;

In[7]:= FindRoot[yd == 71, {f, 300000, 30000, 1.1*10^6}]

FindRoot::frjc: 
   Could not symbolically find the Jacobian of 
    {-71 + 0.0170395 Re[Sqrt[I <<2>>]]}. Try giving two starting values for
     each variable.

                                                    6
Out[7]= FindRoot[yd == 71, {f, 300000, 30000, 1.1 10 }]

> How should I Solve this?

Try giving two starting values for the variable, as suggested
in the warning message.

In[8]:= FindRoot[yd == 71, {f, 30000, 1.1*10^6}]

Out[8]= {f -> 714877.}

This calculation finishes in a lot less than 6 seconds.

Dave Withoff
Wolfram Research



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