Foci is the plural of 'focus'. It is pronounced "FOE - sigh".
Foci of an ellipse
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.
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