k < FF'  iperbole
k > FF'  ellisse

k = 6 (slider from 0 to 8, step 0.1)
F = Point(xAxis)
F': -(F)
D = Point(Circle[F', k])
e = Line(F', D)
a = Segment(F, D)
b = LineBisector(a)
K = Segment(F', D)
C = Intersect(b, e)
poly1, c, a_1, d = Polygon(F, D, C)