Directrix is a line perpendicular to the major axis of the ellipse, which is at a distance a/e from the center of ellipse on either side, where e is the eccentricity of the ellipse.
Eccentricity of ellipse is the ratio of distance of a focus from the center of ellipse to the semi major axis.
Directrix is the line from which the distance of a point P on ellipse and the distance of that point P from a focus F are in a fixed ratio, namely the (1/eccentricity).
b² = a²(1-e²) x²/a² + y²/b² = 1
px+qy+r =0 P = (x1, y1)
(x1-ae)² + y1² = ( p x1 + q y1 + r )² / (p² + q²)