250 CHAPTER 2 Graphs and Functions 73. Student Enrollment In fall 2015, there were 651 students enrolled at the University of the Ozarks. By the fall of 2018, this number had increased to 872. Find and interpret the average annual rate of change in enrollment, to the nearest student, per year for this period. (Data from the University of the Ozarks.) 74. Teen Birth Rates In 1991, there were 61.8 births per thousand for adolescent females aged 15–19. By 2017, this number had decreased to 18.8 births per thousand. Find and interpret the average annual rate of change in teen births per year for this period. Round the answer to the nearest tenth. (Data from U.S. Department of Health and Human Services.) Find the slope of the secant line for each nonlinear function using the two points given. See Example 9. 75. –2 1 2 4 2 0 x y y = f(x) (2, 4) (1, 2) 76. –2 –2 2 1 4 0 y = f(x) x y (0, 5) (2, 1) (Modeling) Solve each problem. See Example 9. 77. Drug Levels in the Bloodstream When a certain drug is taken orally, the amount (in milligrams) of the drug in the bloodstream after t hours is shown in the graph. Find the average rate of change of the amount of the drug in the bloodstream during the time interval from 3 hr to 5 hr. 0 1 2 3 4 5 6 5 10 15 20 25 t y Time (in hours) Amount (in mg) Drug Levels in the Bloodstream (3, 14.5) (5, 4.5) y = f(t) 78. Radioactive Decay A vault holds 25 g of radioactive cobalt. The graph shows the amount remaining after t years. What is the average rate of change of the amount of cobalt remaining from year 7 to year 14? 0 5 10 15 20 5 10 15 20 25 t Time (in years) Amount (in grams) Radioactive Decay (7, 10) (14, 4) y = f(t) y
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