If f is a function of two variables, f (x, y ), its partial derivatives at the point (a, b), denoted by fx (a, b) and fy (a, b), are de?ned by f (a + ?x, b) ? f (a, b) f (a, b + ?y ) ? f (a, b) and fy (a, b) = lim , ?x→0 ?y →0 ?x ...