Re: FindRoot

*To*: mathgroup at smc.vnet.net*Subject*: [mg8812] Re: [mg8793] FindRoot*From*: David Withoff <withoff>*Date*: Fri, 26 Sep 1997 00:33:37 -0400*Sender*: owner-wri-mathgroup at wolfram.com

> 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