In Problems 16-17, show that the function (x, y) does not have a limit as (x, y) - (0, 0). [Hint: Use the line y = mx.] x+y 16. f(x, y) = X - V x2 - 17. f(x, y) = x2+ 120. Show that the function f does not have a limit at (0, 0) by examining the limits of f as (x, y) - (0,0) along the curve y = kx' for different values of k: f (x, y) = x for x * 0 21. Let f (x, y) = V X for x = 0. Is f(x, y) continuous (a) On the x-axis? (b) On the y-axis? (c) At (0, 0)