This term has distinctly different definitions for
Usage 1: For some authors, this refers to the distance from
the center to the focus for either an ellipse or a hyperbola. This definition of focal radius is usually written c.
Usage 2: For other authors, focal radius refers to the
distance from a point on a conic
section to a focus. In this case the focal radius varies depending
where the point is on the curve (unless
the conic in question
is a circle).
are two foci then there are two focal radii.
Note: Using this second definition, the sum of the
focal radii of an ellipse is a constant. It is the same as the
The difference of the focal radii of a hyperbola is a constant.
is the distance between the vertices.
Foci of an ellipse, foci
of a hyperbola, focus of