Foci
Foci is the plural of 'focus'. It is pronounced "FOE  sigh".
Foci of an ellipse
An
ellipse has two foci. The definition of an ellipse is
"A curved line forming a closed loop, where the sum of the distances from two points (foci)
to every point on the line is constant." The two foci always lie on the major axis of the ellipse.
See the links below for animated demonstrations of these concepts.
Definition and properties of an ellipse
How to find the location of the two foci of an ellipse given the ellipse's width and height.
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