gradient vector field

f(x, y) = (x * y)
k = 3.8 (slider from 0 to 5, step 0.1)
g: f(x, y) - k = 0
A = Point(g)
a: x(A)
b: y(A)
f_x = Derivative(f, x)
f_y = Derivative(f, y)
gradient = Vector(A, A + (f_x(a, b), f_y(a, b)))
c: z = k
numericalIntegral1 = SolveODE(f_y(x, y), f_x(x, y), a, b, 4, 0.1)
list1 = Sequence(f(x, y) - k = 0, k, 1, 4, 0.5)