Re: I need some help about this error

*To*: mathgroup at smc.vnet.net*Subject*: [mg39324] Re: I need some help about this error*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Tue, 11 Feb 2003 04:42:14 -0500 (EST)*References*: <b27ft7$nel$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Baruch, The basic problem is that Plot decides what kind of expression {t} is before evaluating {t}. In this case it decides that Flatten[{t}] must be a list of one real number for the sample values of x that it uses. This is not so, in fact Flatten[{t}] becomes a list of several real numbers. We can supply Plot with information to make the correct decision by causing {t} to evaluate before is makes its decision ( I have included f[x] in the plot, and used df = f' (note =, not :=) to avoid evluating f' at each iteration). You might like to consider avoiding the use of Plot for the tangent lines by adding fo each approximation,a, Line[{a,f[a]},{b,0}] where {b,0} is where the tangent at a meets the x-axis -- Epilog might be used. f[x_] := x^2 - 2 Ne[x_] := x - f[x]/f'[x] ap = NestList[Ne, 4., 5] tang[h_] := f[h] + f'[h]*(x - h) t = tang /@ ap Plot[Evaluate[Prepend[t, f[x]]], {x, -5, 5}, AxesOrigin -> {0, 0}, PlotRange -> {-3, f[4]}] Allan --------------------- Allan Hayes Mathematica Training and Consulting Leicester UK www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565 "baruch" <spin9 at terra.com.br> wrote in message news:b27ft7$nel$1 at smc.vnet.net... > f[x_] := x^2 - 2 > Ne[x_] := x - f[x]/f'[x] > ap = NestList[Ne, 1.5, 5] > tang[h_] := f[h] + f'[h](x - h) > t = tang /@ ap > > Plot[{t}, {x, -10, 10}, AxesOrigin -> {0, 0}, > PlotRange -> {{-5, 5}, {-3, 5}}] > > Now, How can I plot the last command? > > I got the following errors: > > Plot::"plnr": "\!\(te[x]\) is not a machine-size real number at > \!\(x\) = \ > \!\(-9.999999166666667`\)." > Plot::"plnr": "\!\(te[x]\) is not a machine-size real number at > \!\(x\) = \ > \!\(-9.188660168541684`\)." > > I want to make a "program" that will plot the successives tangent > lines of approximations of any function (when possible) using the > Newton Methods... any idea on how can I implement or solve that > problem? > > Thank you very much! >

**Follow-Ups**:**Re: Re: I need some help about this error***From:*Dr Bob <drbob@bigfoot.com>