Focal length of an ellipse - urgent help please (1 Viewer)

[davidn92]

Is the focal length of an ellipse the sum of the distances from a point P to the foci (ie, PS + PS' = 2a) or is it the distance between the foci (ie, SS' = 2ae). Reliable sources will be appreciated.

 

[kevin101]

PS + PS' = 2a