Talk:Focus (geometry)

Ellipse[edit]

Shouldn't it be amended as follows:

In an ellipse, the sum of the distances from any point on the ellipse to the two foci is a constant (which is always the length of the ~~semi-~~major axis of the ellipse)

Of course it should; although saving the link takes work. Septentrionalis PMAnderson 23:12, 7 January 2007 (UTC)[reply]

Problem with end of section.[edit]

The line at the end of section 2, "Conics in projective geometry", has all the earmarks of a bad cut-and-paste job.

In projective geometry, all conics are equivalent in the sense that every theorem that can be others.

This sentence does not make logical sense and is not grammatically correct in English.

--Trelligan (talk) 05:53, 12 January 2010 (UTC)[reply]

Parabola definition[edit]

We currently have: An ellipse can be defined as the locus of points for each of which the sum of the distances to two given foci is a constant....A parabola can be defined as a limiting case of an ellipse in which one of the foci is a point at infinity.

The definition of a parabola that follows from this is the locus of points in which the sum of the distance to a focus, and to a point at infinity, is constant. I'm not sure that's helpful. hgilbert (talk) 23:48, 21 September 2011 (UTC)[reply]

The parabola is the limiting case so it can be used as a definition, should be is a different story. I'll rephrase the sentence.--RDBury (talk) 13:57, 22 September 2011 (UTC)[reply]