A = Point(xAxis)
B = Point(xAxis)
C = Midpoint(A, B)
a = OrthogonalLine(C, xAxis)
S = Point(a)
f(x) = (x - x(A)) * (x - x(B)) * 4.0 * y(S) / (-1.0 * x(A)^(2.0) + 2.0 * x(A) * x(B) - x(B)^(2.0))
Text1: "f(x) = " + (Simplify[f(x)])