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2)Gottingen's Max Wardetzky, Ingo Witt, are you as dumb and messy about Conics as Franz? Oval is the conic sect ion, never ellipse
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Last Post:
Oct 6, 2017 3:00 PM



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Re: 2)Gottingen's Max Wardetzky, Ingo Witt, are you as dumb and messy about Conics as Franz? Oval is the conic section, never ellipse
Posted:
Oct 6, 2017 2:57 PM


On Friday, October 6, 2017 at 10:54:16 AM UTC+2, Archimedes Plutonium wrote: > Franz wrote: > > > > ...that certain cone sections as well as certain cylinder sections are > > ellipses. > >
Right. You will find a simple proof for that claim below.
Some preliminaries:
Top view of the conic section and depiction of the coordinate system used in the proof:
^ x  + <= x=h .'  `. /  \ .  .    .  . \  / `.  .´ y <+ <= x=0 Cone (side view): . /\ /  \ /b  \ /+´ <= x = h / ´ \ / ´  \ / ´  \ x = 0 => ´+\ / a  \
Proof:
r(x) = a  ((ab)/h)x and d(x) = a  ((a+b)/h)x, hence
y(x)^2 = r(x)^2  d(x)^2 = ab  ab(2x/h  1)^2 = ab(1  4(x  h/2)^2/h^2.
Hence (1/ab)y(x)^2 + (4/h^2)(x  h/2)^2 = 1 ...equation of an ellipse
qed
======================================================
@Archie: Yes, this proves that (certain) cone sections "as depicted in my diagram" are ellipses.
Still not convinced? Can you point out an error in my simple calculation (of the shape of the coneic section) above?



