7. Explain why a maximum value for the function f ( x, y, z) = - 2x- z' + cos (2x + 2y - 3= + 1) must occur at points where x"+ y'+ z* $ 4. Then use random guesses to determine the critical values of f where x" + y' + z'S 4. Determine which of these are points where f has a local maximum, and which are points where f has a local minimum. Finally, evaluate f at these points to determine the maximum value of f