Ellipse
An ellipse is a type of conic section.
Contents
Definition
An ellipse is formed by cutting through a cone at an angle. Equivalently, it is defined as the locus, or set, of all points such that the sum of the distances from to two fixed foci is a constant. (The equivalence of these two definitions is a nontrivial fact.)
Intuition
Ellipses tend to resemble circles which have been "flattened" or "stretched." They occur in nature as well as in mathematics: as was proven in Kepler's Laws, the planets all revolve about the sun in elliptical, not circular, orbits with the sun at one of the foci. Note that the circle is just a special case of the ellipse, just as a square is to a rectangle, and occurs when (in the first definition) the cutting plane is perpendicular to the axis of the the cone, or (in the second definition) the two foci of the ellipse coincide.
Using the second definition of an ellipse given above, one may easily construct an ellipse from household materials. To draw an ellipse with two pushpins, a loop of string, pencil, and paper, stick the pushpins in the paper place the string on the paper so that both pushpins are inside it. The pushpins will be the foci of the ellipse, and the length of the string will determine the sum of the distances from a point on the ellipse to the two foci. Hold the pencil on the paper such that the string is taut against the pencil tip and the two pushpins. Then move the pencil tip while keeping the string taut. This will trace out an ellipse.
Related Terminology
For a given noncircular ellipse, there will be two points on the ellipse closest to the center and two points furthest away  it will be "tall and skinny" or "short and fat." The segment connecting the center of the ellipse to one of the "farther away ends" is called the semimajor axis and the segment connecting the center to a closer end is called the semiminor axis. These two segments are perpendicular. Drawing all four semiaxes divides the ellipse into 4 congruent quarters.

Ellipse 
An ellipse in a Cartesian coordinate system with center whose axes are parallel to the coordinate axes, with the horizontal semiaxis of length and the vertical semiaxis of length is given by the equation . In particular, if the center of the ellipse is the origin this simplifies to .
The threedimensional counterpart of the ellipse is the ellipsoid.
Properties
 Let be the foci of an ellipse, let be a point on the ellipse, and let be the tangent line to the ellipse at . Then it is true that the bisector of angle is perpendicular to .
Related Formulae
 The area of an ellipse with semimajor and semiminor axes is .
 The circumference of an ellipse is , where the is the second elliptic integral.
Problems
Introductory
 The ellipse with axis lengths and has the general equation of . Find the value of .
Intermediate
 Let and denote the circles and respectively. Let be the smallest possible value of for which the line contains the center of a circle that is externally tangent to and internally tangent to Given that where and are relatively prime integers, find (Source)
 An equilateral triangle is inscribed in the ellipse whose equation is . One vertex of the triangle is , one altitude is contained in the yaxis, and the length of each side is , where and are relatively prime positive integers. Find . (Source)
 An ellipse has foci at and in the plane and is tangent to the axis. What is the length of its major axis? (Source)