Find the derivative of the function f(x, y) = arctan(y/x) at point (-9, 9) in the direction the function increases most rapidly.
a) ∂f/∂x = 1/18; ∂f/∂y = -1/18
b) ∂f/∂x = 1/9; ∂f/∂y = -1/9
c) ∂f/∂x = -1/9; ∂f/∂y = 1/9
d) ∂f/∂x = -1/18; ∂f/∂y = 1/18