[M2.3]
of: a given conic section
is: the ratio of the distance PF from any point P on the conic section to a focus F of the conic section, to the perpendicular distance PD from the point P to the directrix (i.e. [M2.3]
is exemplified: by the eccentricity 
of an ellipse for which 
and the lengths of the semi-major axis 
and the semi-minor axis 
are related by 
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