Auxiliary Circle and Ellipse

Auxiliary Circle and Ellipse


The auxiliary circle is a construction associated with an ellipse that aids in understanding its geometry and properties. It is a circle used to derive the equation of an ellipse and provides additional insights into its characteristics.


  1. Deriving the Equation of an Ellipse:

    • The auxiliary circle assists in obtaining the standard equation of an ellipse by geometric constructions involving the relationship between the ellipse and its auxiliary circle.
  2. Understanding Ellipse Parameters:

    • Helps in visualizing and defining ellipse parameters such as semi-major axis, semi-minor axis, and foci.

Construction of the Auxiliary Circle:

  1. Standard Form of the Ellipse:

    • The equation of an ellipse in standard form is: (xh)2a2+(yk)2b2=1
  2. Construction Steps:

    • Consider an ellipse centered at (h,k) with semi-major axis a and semi-minor axis b.
    • Create a circle centered at the same point (h,k) with a radius equal to the semi-major axis length a.

Relationship between Ellipse and Auxiliary Circle:

  1. Geometry:

    • The intersection points between the ellipse and the auxiliary circle correspond to the vertices of the ellipse along its major axis.
  2. Foci and Center:

    • The foci of the ellipse are located on the major axis, lying within the auxiliary circle at a distance of c units from the center.
    • The center of the ellipse coincides with the center of the auxiliary circle.
  3. Derivation of Ellipse Equation:

    • Geometrically, the equation of the ellipse can be derived by considering the distances between points on the ellipse and the foci, utilizing properties of the auxiliary circle.