Iff: 2- is defined by f(x, y) = xy/(x + y) unless x that D.f(0, 0) exists for all v, but f is not differentiable at (0, 0). Hint: Note first that f(tv) = tf(v) for all te and ve . Then show that D.f(0, 0) = f(v) for all v. Hence y = 0, and f(0, 0) = 0, show Dif(0,0) - Df(0, 0) = 0 but D(1, 1)f(0, 0) = 4.