An ellipse is the trajectory of a moving point P. The sum of the distances from the moving point P to the fixed points F 1 and F2 in a plane is equal to a constant (greater than |F 1F2|), and F1and F2 are called the two focuses of the ellipse. The mathematical expression is | pf1|+pf2 | = 2a (2a > | f1F2 |).

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An ellipse is a closed cone section: a plane curve intersects a plane through a cone. There are many similarities between the other two forms of ellipse and cone section: parabola and hyperbola, which are both open and unbounded. The cross section of a cylinder is elliptical unless it is parallel to the axis of the cylinder.

An ellipse can also be defined as a set of points, so that the ratio of the distance between each point on the curve and a given point (called focus) to the distance between the same point on the curve (called directrix) is a constant. This ratio is called eccentricity of ellipse.