Foci (focus points) of an ellipse

From  http://www.mathopenref.com/ellipsefoci.html

where F  is the distance from each focus to the center (see figure below) j  is the semi-major axis (major radius) n  is the semi-minor axis (minor radius) By definition, a+b always equals the major axis length QP, no matter where R is. So a+b equals OP+OQ. So b must equal OP. (And a equals OQ).