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Topic: Re: [mg4908] help!! Plot3D of ellipsoid
Replies: 0

 Keith S. Mersman Posts: 10 Registered: 12/7/04
Re: [mg4908] help!! Plot3D of ellipsoid
Posted: Oct 9, 1996 2:07 PM

On Fri, 4 Oct 1996, Seth wrote:

> I am trying to get a 3-d plot of an ellipsiod, of the equation
>
> (x^2/a^2) + (y^2/b^2) + (z^2/c^2) = 1
>
> manipulating the equation I can get
>
> z = (c^2 (1- (x^2/a^2) - (y^2/b^2)))^1/2
>
> then, with the constants defined, I write
>
> Plot3D[z,{x,0,a},{y,0,b}]
>
> Mathematica then spits out a bunch of errors
>
>
>

Seth,

The reason you're getting error messages is because your function z does
not have real values for the square {0,a}X{0,b}. It is only defined on an
elliptical region in that area. The syntax of Plot3D, however, designates
that it must be defined on a rectangular region. To get around this, you
can use modified spherical coordinates.
{xr_,[s_,t_],y[r_,s_,t_],z[r_,s_,t_]}={a r Sin[s] Cos[t], b r Sin[s]
Sin[t], c r Cos[s]}. Setting r equal to 1, you can verify that x^2/a^2 +
y^2/b^2 + z^2/c^2 = 1. Then you can use
ParametricPlot3D[{x[1,s,t],y[1,s,t],z[1,s,t]},{s,0,Pi},{t,0,2 Pi}]

to plot the ellipsoid.

======================================================
Keith S. Mersman
Mathematica Consultant,
University of Missouri-Columbia

Mailbox: 220 Math Science Building
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http://www.phlab.missouri.edu/~mersman
and
http://www.missouri.edu/~c621746
======================================================