The cubic equation for the four-point rectangle is developed as described
above for quadratic Eq. (B). Let u(P) = P3. Now assign A=u(1), B=u(3), C=u(7),
D=u(9) and F=u(1+t), G=u(3+t), H=u(7+t), I=u(9+t) in Fig. 1. Apply the cubic
equation in [3] to the preceding data and set z = –1 in the equation. Then take the
limit of the expression as t→0. The result is a cubic equation for the four-point
rectangle ABDC in Fig. 1. That equation is R = (5+x+3y)3.