'The focal distance of the ellipse.
The distance between the center and one focus.
Returns
focus_distance : number
See Also
foci
Examples
>>> from sympy import Point, Ellipse
>>> p1 = Point(0, 0)
>>> e1 = Ellipse(p1, 3, 1)
>>> e1.focus_distance
2*sqrt(2)'

@property
def focus_distance(self):

    return Point.distance(self.center, self.foci[0])
