f_1(x, y) = sqrt(x^(2) + y^(2))
f_2(x, y) = (-sqrt(x^(2) + y^(2)))
A = (-2, 2, 0)
Z_A: (-2, 2, f_1(-2, 2))
Z_A2: (-2, 2, f_2(-2, 2))