\fx2ky3 cos(x2 + y? ) , if (a, y) # (0, 0) f(x, y) = 26 + yb 0 , if (x, y) = (0,0) where k is a positive constant. (a) Find all values of k for which f is continuous at (0, 0). In other words, find all values of k for which 2k y3 cos(x2 + y? ) lim(x,y)-+0 = 0. x6 + yoJustify your answer. Hint: I think your expression will "simplifv" if vou use polar coordinates m = r' (308(3) and y = r' 3111(3). ii; (b) Recall that the partial derivative of fwith respect toy at a point (a, b) in the domain of f is farm, 5) : limHu w. Using our expression for f, find all values of k for which the partial derivative 310,15) 3 Jay\"): 0) = limit>0 exists. For those values of Is. what is the value fy (U, D}