Re: I can plot a function but when I try FindRoot it

*To*: mathgroup at smc.vnet.net*Subject*: [mg92697] Re: [mg92678] I can plot a function but when I try FindRoot it*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Sat, 11 Oct 2008 06:41:12 -0400 (EDT)*Reply-to*: hanlonr at cox.net

You have a bad definition for k and should restrict myDs to numeric arguments. myDs[p_?NumericQ] := FindRoot[f1[Ltotal*p, ds] - 1, {ds, 2.5}][[1, 2]] Bob Hanlon ---- Toan <nguyenthetoan at gmail.com> wrote: ============= Hi all, I have a problem with Mathematica error that I don't know how to fix. I define a function f2 [L,ds(L)] where ds(L) is found by solving f1(L,ds)==1. When I plot f2 [L, ds(L)] as a function of L, everything is good. However, when I try to find the value of L where f2 ==1, Mathematica complains that f2 is not a number?! The mathematica cell is listed below. Could someone help me with this error? I thank you in advance, a lot. Toan ================================= k5.62; Ro = 27.2; Ltotal = 13509.0; F0 = 1.2*10^4; c = 0.30; R[L_, ds_] = Sqrt[Ro^2 - ((3 Sqrt[3])/(8 Pi) ds^2 L)^(2/3)]; f1[L_, ds_] := (k/(R[L, ds]^2 ds^2) + 3 k/ ds^2 (1/((Ro^2) - (R[L, ds]^2) ) + Ro/(Ro^2 - R[L, ds]^2)^(3/2) Log[(Ro - Sqrt[Ro^2 - R[L, ds]^2])/R[L, ds]]))/(Sqrt[3] F0 Exp[-ds/c]); f2[L_, ds_] := (Sqrt[3] F0 Exp[-(ds/c)] (c^2 + c ds) + k/( 2 (R[L, ds]^2) ))/(Pi); myDs[p_] := ( result = FindRoot[f1[Ltotal*p, ds] - 1, {ds, 2.5}]; result[[1, 2]] ) Plot[f2[Ltotal*p, myDs[p]], {p, 0.01, 1.0}, PlotRange -> {0, 1}, Frame -> True] FindRoot[ f2[Ltotal*p, myDs[p] ]-1, {p,0.8}] -- Bob Hanlon