a = 1 (slider from 0.1 to 5, step 0.1)
p: y² = 4 a x
A_1 = Point(p)
C = Intersect(xAxis, yAxis)
e = Line(A_1, C)
f = OrthogonalLine(C, e)
A_2 = Intersect(p, f)
g = Segment(A_1, A_2)
E = Intersect(g, xAxis)