Editing Ellipse (section)

==== Semi-latus rectum ====
The length of the chord through one focus, perpendicular to the major axis, is called the ''latus rectum''. One half of it is the ''semi-latus rectum'' <math>\ell</math>. A calculation shows:<ref>{{harvtxt|Protter|Morrey|1970|pp=304,APP-28}}</ref>
<math display="block">\ell = \frac{b^2}a = a \left(1 - e^2\right).</math>

The semi-latus rectum <math>\ell</math> is equal to the [[radius of curvature]] at the vertices (see section [[#Curvature|curvature]]).