# Algebra/Ellipse

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An ellipse is the collection of points that are equidistant from two points, called *foci* (singular *focus*).

The foci are found on the **major axis**, which has a length of 2a. The **minor axis** is 2b, and is smaller.

The "roundness" or "longness" of an ellipse can be measured by eccentricity. If c is the distance from the center to a focus, then e = c / a.

The **latus rectum** is a line parallel to the minor axis that crosses through a focus. Its length is b^{2} / a.

"Long" ellipses are generally written as

where (h,k) is the center, while "tall" ellipses are written as