Esc F1 SECTION 5.5 The Fundamental Theorem of Calculus 379 where t is the number of years that the mine has been in operation. Find the useful life of the mine, to the nearest year. What is the total profit accumulated during the useful life of the mine? 77. Average cost. The total cost (in dollars) of manufacturing x auto body frames is C(x) = 60,000 + 300x. bos 8 = p (A) Find the average cost per unit if 500 frames are produced. [Hint: Recall that C(x) is the average cost per unit. ] (B) Find the average value of the cost function over the $ interval [0, 500]. Inwollof sri m nworle at toov sidi (C) Discuss the difference between parts (A) and (B). 78. Average cost. The total cost (in dollars) of printing x dic- tionaries is C(x) = 20,000 + 10x. (A) Find the average cost per unit if 1,000 dictionaries are produced. (B) Find the average value of the cost function over the interval [0, 1,000]. (C) Discuss the difference between parts (A) and (B). 79. Sales. The rate at which the total number of sales is chang- ing is given by S' (t) where t is the number of months since the product's release. What does represent? 80. Advertising. The rate at which the total amount of money dvertising campaign is changing is given by ampaign