Geometric figure in which the sum of the distances of each point

*P*from two fixed points*F*_{1}and*F*_{2}, called the foci, is constant.- Consider an ellipse with foci
*F*_{1}and*F*_{2}and points*P*_{1}and*P*_{2}:

d(P\(_{1}\), F\(_{1}\)) + d(P\(_{1}\), F\(_{2}\)) = d(P\(_{2}\), F\(_{1}\)) + d(P\(_{2}\), F\(_{2}\))

- Segments
*a*and*b*in the illustration are the semi-minor axis (conjugate axis) and the semi-major axis (transverse axis) of the ellipse. The major axis (transverse axis) is the diameter of the ellipse. - An ellipse can also be formed by the intersection of a cone of revolution with a plane that meets all the generating lines and that does not pass through the apex.

### Formula

The area *A* of an ellipse of semi-axes *a* and *b* is : *A* = π*ab*.