
 Ellipse 
  An ellipse can be represented parametrically by the equations x = a cos θ and y = b sin θ, where x and y are the rectangular coordinates of any point on the ellipse, and the parameter θ is the angle at the center measured from the xaxis anticlockwise. 
  Therefore, a normal to the ellipse is found by bisecting the angle between the radii to F and F', and a tangent is perpendicular to the normal. 
  The focal definition of an ellipse is that an ellipse is the locus of points the ratio of whose distances from a fixed line, the directrix, to a fixed point, the focus, is a constant e, called the eccentricity. 
 www.du.edu /~jcalvert/math/ellipse.htm (5042 words) 
