It may be interesting to discuss the construction as an introduction to conic sections:
d = |DC| and k = |DF| are given.
[math]h=\frac{d^2+k^2}{2k}[/math] is then the distance from a point, P, on the parabola to each of C and F.
The formula for h is easily found by considering the triangle PGF.
PQ is tangential to the parabola.
