3847. Second partial derivatives Find the four second partial derivatives of the following functions. 40. f(x, y) = 2x y + xy 48-53. Equality of mixed partial derivatives Verify that fxy = fyx for the following functions. 50. f(x, y) = xy 77-78. Nondifferentiability? Consider the following functions f. 78. 2xy2 f(x, y) = x+y4 if (x, y) (0,0) 0 if (x, y) = (0, 0)