16. Consider the function 3:3 . f2R2)R:(1:,y)I) 332+y2 lf(a:,y)7(0,0), 0 if (3:, y) = (0, 0)- (a) Verify that f is di'erentiable in (0, 0) according to any direction a and also determine the directional derivative at (0, 0) in the direction of u. (b) Show that f nevertheless has no rst order approximation in (U, U). 17. Consider the function g : R2 } R that has continuous partial derivatives at least up to second order. Consider the function f : R2 > R : (may) I> sin(3m)g(2y,a:y). Determine D12 f (1:, 3;) expressed in terms of the partial derivatives of g