3. a) Explain how you know for certain that f(x, y) = xy xy has an absolute maximum and an absolute minimum over the region R bounded by 0 x 1, 0 y 1. b) Find the absolute maxima/minima of f(x, y) = xy xy over the region R. 4. Find the absolute maxima/minima of the function f(x, y) = x + xy + y over the region R = {(x, y) = Rx + y 1}.