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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
>
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