'The focal length of the parabola.
Returns
focal_lenght : number or symbolic expression
Notes
The distance between the vertex and the focus
(or the vertex and directrix), measured along the axis
of symmetry, is the "focal length".
See Also
https://en.wikipedia.org/wiki/Parabola
Examples
>>> from sympy import Parabola, Point, Line
>>> p1 = Parabola(Point(0, 0), Line(Point(5, 8), Point(7, 8)))
>>> p1.focal_length
4'

@property
def focal_length(self):

    distance = self.directrix.distance(self.focus)
    focal_length = (distance / 2)
    return focal_length
