plot ellipsoidal normal

[Clara Brück]

Hello,

I've got an ellipsoid as a 3D-Graph:

#Ellipsoid
xc=0
yc=0
zc=0
xr=yr= 7000
zr=5000
n = 25
[A,B,C] = ellipsoid(xc,yc,zc,xr,yr,zr,n);
mesh(A,B,C)

Then I have a Point on the surface.:

p2= [A(17,20) B(17,20) C(17,20)];

How can I plot a line, that goes orthogonal to the surface in this point, to 
the middle (z-axis)?

Hope someone's got an idea.

Greetings
Clara