xty 28. Prove that arctan r + arctany = arctan 1 - xy 29. Consider the function 2 3 f : R' - R : (x,y) f(x,y) = 9 1 2-x-y 3+ 2x +2y ), 2x - 2y x- y 2x - 2y with g : R3x3 _> R a function that is linear and alternating in the rows, and for which -1 2 g 1 5 = 3. 4 Determine the gradient of f