Re: Plotting

*To*: mathgroup at smc.vnet.net*Subject*: [mg72995] Re: [mg72987] Plotting*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Sun, 28 Jan 2007 00:47:50 -0500 (EST)*Reply-to*: hanlonr at cox.net

f[x_]:=x^3-16 x^2+3 x+1; rts=Simplify[x/.Solve[f[x]==0,x]] {16/3 + (13*19^(2/3))/(3*((1/2)*(407 + 21*I*Sqrt[3]))^(1/3)) + (1/3)*((19/2)*(407 + 21*I*Sqrt[3]))^(1/3), 16/3 - (13*(19/2)^(2/3)*(1 + I*Sqrt[3]))/ (3*(407 + 21*I*Sqrt[3])^(1/3)) + (1/6)*I*((19/2)*(407 + 21*I*Sqrt[3]))^(1/3)*(I + Sqrt[3]), 16/3 - (13*(19/2)^(2/3)*(1 - I*Sqrt[3]))/ (3*(407 + 21*I*Sqrt[3])^(1/3)) - (1/6)*(1 + I*Sqrt[3])* ((19/2)*(407 + 21*I*Sqrt[3]))^(1/3)} rts//N//Chop {15.8062,-0.172647,0.366449} Simplify[f/@rts] {0,0,0} Plot[f[x],{x,-1,20}, Epilog->{Red,AbsolutePointSize[5], Point/@({#,0}&/@rts)},ImageSize->500]; Bob Hanlon ---- Rayne <ghcgwgwg at singnet.com.sg> wrote: > Hi all, > I want to plot the cubic equation f(x) == x^3 - 16x^2 + 3x + 1 == 0. I had: > > f==x^3-16 x^2+3 x+1 ==== 0 > Plot[f,{x,-1,20}] > > but I was unable to get the plot. The error message says > > Plot::plnr: f is not a machine-size real number at x == -0.999999. > Plot::plnr: f is not a machine-size real number at x == -0.148093. > Plot::plnr: f is not a machine-size real number at x == 0.780985. > General::stop: Further output of Plot::plnr > will be suppressed during this calculation. > > How do I get around this problem? > > Thank you. > > Regards, > Rayne >