Let C(q) represent the cost, R(q) the revenue, and (q) the total profit, in dollars, of producing q items.
(a) If C'(50) = 75 and R'(50) = 84, approximately how much profit is earned by the 51st item?
(b) If C'(90) = 71 and R'(90) = 68, approximately how much profit is earned by the 91st item?
(c) If (q) is a maximum when q = 78, how do you think C'(78) and R'(78) compare? Explain.