The manufacturing cost (in dollars) for a certain computer is given by c(x, y) = 2x + y² + 4xy + 25, where x is the memory capacity of the computer in gigabytes (GB) and y is the number of hours of labor required. For 640 GB and 6 hours of labor, find the following.
(a) The approximate change in cost for an additional 1 GB of memory
(b) The approximate change in cost for an additional hour of labor