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Topic: construction of triangle of given perimeter, given point and angle
Replies: 16   Last Post: Jun 10, 2011 12:58 PM

 Messages: [ Previous | Next ]
 Peter Ash Posts: 13 Registered: 12/6/04
Re: parabola through 4 points
Posted: Nov 17, 1997 8:53 PM

Michael,

though.

Michael Keyton wrote:
>
> Peter has asked about finding the foci of a hyperbola knowing 5 points
> that lie on the hyperbola. I can not fully solve this, but I can find the
> center of the hyperbola. For the ellipse, I can find both the center and
> the hyperbola, but it is not yielding itself to me quickly.
> Ellipse: Construct any chord of the ellipse, construct a parallel chord.
> The line through the midpoints of two parallel chords passes through the
> center. (I do not know a proof of this without adding a point at infinity
> and looking at poles. I can give a reasonable argument using symmetry of
> the ellipse, but it lacks the proper rigor.) Repeat for a second chord,
> the interesection of the two lines is the center of the ellipse. This also
> works for finding the center of teh hyperbola.

I agree that the line through the midpoints of two parallel chords of
any
conic passes through the center of the conic. (Proof: The projective
transformation
that fixes the center of the conic and maps it to a circle takes
segments to segments, parallel lines to parallel lines, and midpoints of
segments to midpoints of segments.) But given five arbitary points on a
conic, how do you get two parallel chords? Am I missing something
obvious?

>
> Construct any circle centered at this point that intersects the ellipse in
> 4 points (use the five points, 3 can't be the vertices of the minor axis.)
> The perpendicular bisectors of the consecutive pairs of points are the
> major and minor axis. (In the hyperbola case, this will find the axis for
> the foci) Use compass and straightedge, knowing the relationships between
> the distance between the foci and the center and the semi-major and minor
> axis lengths fill find the foci length. This can be constructed easily.

How do you locate the points of intersection between a circle centered
at
the center of the conic and the conic? And then how do you find the
intersections of the axes with the ellipse? Once you've got that, of
course you are right, its easy to find the foci.

>
> Unfortunately, for the hyperbola where is the length of the minor axis?
>

It would seem you would have to know the asymptotes.

> Michael Keyton
> St. Mark's School of Texas
>

Date Subject Author
11/7/97 Anatoly Levin
11/7/97 Joshua Zucker
11/7/97 John Conway
11/7/97 Joshua Zucker
11/7/97 Michael Keyton
11/8/97 Jon Roberts
11/8/97 John Conway
11/11/97 Michael Thwaites
11/11/97 Eileen M. Klimick Schoaff
11/11/97 Michael Keyton
11/11/97 Eileen M. Klimick Schoaff
11/11/97 John Conway
11/11/97 Peter Ash
11/12/97 Michael Keyton
11/17/97 Peter Ash
6/10/11 Fred
12/5/97 Jon Roberts