Focus - Directrix Parabola Definition
Author: cwl

The parabola as the locus of points equidistant from 
a point (A) (the focus) and a line (b) (the directrix).

a = 2 (slider from 0 to 10, step 0.1)
A = (0.16, 1.44)
B = (-2.42, -0.18)
C = (4.66, -0.18)
b = Line(B, C)
c = Circle(A, a)
d = OrthogonalLine(A, b)
D = Intersect(b, d)
e = Circle(D, a)
E = Intersect(e, d, 2.0)
f = OrthogonalLine(E, d)
F = Intersect(c, f, 1.0)
G = Intersect(c, f, 2.0)
g = Segment(A, F)
h = OrthogonalLine(F, b)
H = Intersect(b, h)
i = Segment(F, H)
j = Segment(A, G)
k = OrthogonalLine(G, b)
I = Intersect(b, k)
l = Segment(G, I)