A=(x,y)

z=f(x,y)

B=(x,y,f(x,y))

A in der Grundebene 
mit den Pfeiltasten umherschieben

a(x, y) = If[x > 0 ∧ x < 3 ∧ y > 0 ∧ y < 3, 1 + cos((2 * y)) / 3 - cos((2 * x)) / 3]
A = (1.9999999999999996, 0.5, 0)
B: (x(A), y(A), a(x(A), y(A)))
f = Segment(B, A)
C: (0, y(A))
g = Segment(C, A)
D: (x(A), 0)
h = Segment(A, D)
E: (0, 0, z(B))
i = Segment(E, B)
F = (-3.348, 3.868)
G = (-1.648, 3.868)