1. College Algebra & Trigonometry
and
Precalculus
4th EDITION

2. 4.6 Applications and Models of Exponential Growth and Decay
The Exponential Growth or Decay Function
Growth Function Models
Decay Function Models

3. Exponential Growth or Decay Function
In many situations that occur in ecology, biology, economics, and the social sciences, a quantity changes at a rate proportional to the amount present. In such cases the amount present at time t is a special function of t called an exponential growth or decay function.

4. Exponential Growth or Decay Function
Let y0 be the amount or number present at time t = 0. Then, under certain conditions, the amount present at any time t is modeled by
where k is a constant.

5. Exponential Growth or Decay Function
When k > 0, the function describes growth; in Section 4.2, we saw examples of exponential growth: compound interest and atmospheric carbon dioxide.
When k < 0, the function describes decay; one example of exponential decay is radioactivity.

6. Year Carbon Dioxide (ppm)
DETERMINING AN EXPONENTIAL FUNCTION TO MODEL THE INCREASE OF CARBON DIOXIDE
Example 1
In Example 11, Section 4.2, we discussed the growth of atmospheric carbon dioxide over time. A function based on the data from the table was given in that example. Now we can see how to determine such a function from the data.
Year
Carbon Dioxide (ppm)
1990
353
2000
375
2075
590
2175
1090
2275

7. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL THE INCREASE OF CARBON DIOXIDE
Example 1
Find an exponential function that gives the amount of carbon dioxide y in year t. a.
Solution
Recall that the graph of the data points showed exponential growth, so the equation will take the form y = y0ekt. We must find the values of y0
and k. The data began with the year 1990, so to simplify our work we let 1990 correspond to t = 0, 1991 correspond to t = 1, and so on. Since y0
is the initial amount, y0 = 353 in 1990, that is, when t = 0.

8. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL THE INCREASE OF CARBON DIOXIDE
Example 1
Find an exponential function that gives the amount of carbon dioxide y in year t. a.
Solution
Thus the equation is
From the last pair of values in the table, we know that in 2275 the carbon dioxide level is expected to be 2000 ppm. The year 2275 corresponds to 2275 – 1990 = Substitute 2000 for y and 285 for t and solve for k.

9. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL THE INCREASE OF CARBON DIOXIDE
Example 1
Solution
Divide by 353.
Take logarithms on both sides.
In ex = x

10. ∙ Solution Example 1 A function that models the data is
DETERMINING AN EXPONENTIAL FUNCTION TO MODEL THE INCREASE OF CARBON DIOXIDE
Example 1
Solution ∙
Multiply by ; rewrite.
Use a calculator.
A function that models the data is

11. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL THE INCREASE OF CARBON DIOXIDE
Example 1 b.
Estimate the year when future levels of carbon dioxide will double the 1951 level of 280 ppm.
Solution
Let y = 2(280) = 560 in y = 353e.00609t , and find t.
Divide by 353.
Take logarithms on both sides.

12. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL THE INCREASE OF CARBON DIOXIDE
Example 1
In ex = x ∙
Multiply by ;
Rewrite.
Use a calculator.
Since t = 0 corresponds to 1990, the 1951 carbon dioxide level will double in the 75th year after 1990, or during 2065.

13. Solution FINDING DOUBLING TIME FOR MONEY Example 2
How long will it take for the money in an account that is compounded continuously at 3% interest to double?
Solution
Continuous compounding formula
Let A = 2P and r = .03
Divide by P
Take logarithms on both sides.

14. Solution FINDING DOUBLING TIME FOR MONEY Example 2
How long will it take for the money in an account that is compounded continuously at 3% interest to double?
Solution
In ex = x
Divide by .03
Use a calculator.
It will take about 23 yr for the amount to double.

15. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL POPULATION GROWTH
Example 3
According to the U.S. Census Bureau, the world population reached 6 billion people on July 18, 1999, and was growing exponentially. By the end of 2000, the population had grown to billion. The projected world population (in billions of people) t years after 2000, is given by the function defined by

16. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL POPULATION GROWTH
Example 3 a.
Based on this model, what will the world population be in 2010?
Solution
Since t = 0 represents the year 2000, in 2010, t would be 2010 – 2000 = 10 yr. We must find (t) when t is 10.
Let t = 10.
According to the model, the population will be billion at the end of 2010.

17. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL POPULATION GROWTH
Example 3 b.
In what year will the world population reach 7 billion?
Solution
Let t = 7.
Divide by 6.079
Take logarithms on both sides

18. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL POPULATION GROWTH
Example 3 b.
In what year will the world population reach 7 billion?
Solution
In ex = x
Divide by .0126; rewrite.
Use a calculator.
World population will reach 7 billion 11.2 yr after 2000, during the year 2011.

19. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL RADIOACTIVE DECAY
Example 4
If 600 g of a radioactive substance are present initially and 3 yr later only 300 g remain, how much of the substance will be present after 6 yr?
Solution
To express the situation as an exponential equation
we use the given values to first find y0 and then find k.

20. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL RADIOACTIVE DECAY
Example 4
If 600 g of a radioactive substance are present initially and 3 yr later only 300 g remain, how much of the substance will be present after 6 yr?
Solution
Let y = 600 and t = 0.
e0 = 1
Thus, y0 = 600 and the exponential equation y = y0ekt becomes

21. Solution Example 4 Now solve this exponential equation for k.
DETERMINING AN EXPONENTIAL FUNCTION TO MODEL RADIOACTIVE DECAY
Example 4
Solution
Now solve this exponential equation for k.
Let y = 300 and t = 3.
Divide by 600.
Take logarithms on both sides.

22. DETERMINING AN EXPONENTIAL FUNCTION TO MODEL RADIOACTIVE DECAY
Example 4
Solution
In ex = x
Divide by 3.
Use a calculator.

23. Solution Example 4 Thus, the exponential decay equation is
DETERMINING AN EXPONENTIAL FUNCTION TO MODEL RADIOACTIVE DECAY
Example 4
Solution
Thus, the exponential decay equation is
y = 600e –.231t . To find the amount present after 6 yr, let t = 6.
After 6 yr, about 150 g of the substance will remain.

24. Half Life
Analogous to the idea of doubling time is half-life, the amount of time
that it takes for a quantity that decays exponentially to become half its initial
amount.

25. SOLVING A CARBON DATING PROBLEM
Example 5
Carbon 14, also known as radiocarbon, is a radioactive form of carbon that is found in all living plants and animals. After a plant or animal dies, the radiocarbon disintegrates. Scientists can determine the age of the remains by comparing the amount of radiocarbon with the amount present in living plants and animals. This technique is called carbon dating. The amount of radiocarbon present after t years is given by
where y0 is the amount present in living plants and animals.

26. a. Solution SOLVING A CARBON DATING PROBLEM Example 5
Find the half-life.
Solution
If y0 is the amount of radiocarbon present in a living thing, then ½ y0 is half this initial amount. Thus, we substitute and solve the given equation for t.
Let y = ½ y0 .

27. a. Solution SOLVING A CARBON DATING PROBLEM Example 5
Find the half-life.
Solution
Let y = ½ y0 .
Divide by y0.
Take logarithms on both sides.

28. a. Solution SOLVING A CARBON DATING PROBLEM Example 5
Find the half-life.
Solution
In ex = x
Divide by –
Use a calculator.
The half-life is about 5700 yr.

29. b. Solution SOLVING A CARBON DATING PROBLEM Example 5
Charcoal from an ancient fire pit on Java contained ¼ the carbon 14 of a living sample of the same size. Estimate the age of the charcoal.
Solution
Solve again for t, this time letting the amount
y = ¼ y0.
Given equation.
Let y = ¼ y0 .

30. Solution SOLVING A CARBON DATING PROBLEM Example 5
Divide by y0 .
Take logarithms on both sides
In ex = x; divide by −
Use a calculator.
The charcoal is about 11,400 yr old.

31. MODELING NEWTON’S LAW OF COOLING
Example 6
Newton’s law of cooling says that the rate at which a body cools is proportional to the difference C in temperature between the body and the environment around it. The temperature (t) of the body at time t in appropriate units after being introduced into an environment having constant temperature T0 is
where C and k are constants.

32. a. Solution MODELING NEWTON’S LAW OF COOLING Example 6
A pot of coffee with a temperature of 100°C is set down in a room with a temperature of 20°C. The coffee cools to 60°C after 1 hr. a.
Write an equation to model the data.
Solution
We must find values for C and k in the formula for cooling. From the given information, when t = 0, T0 = 20, and the temperature of the coffee is (0) = Also, when t = 1, (1) = 60. Substitute the first pair of values into the equation along with T0 =20.

33. a. Solution MODELING NEWTON’S LAW OF COOLING Example 6
Write an equation to model the data.
Solution
Given formula
Let t = 0, (0) = 100, and T0 = 20.
e0 = 1
Subtract 20.
Thus,

34. Solution MODELING NEWTON’S LAW OF COOLING Example 6
Now use the remaining pair of values in this equation to find k.
Given formula
Let t = 1, (1) = 60.
Subtract 20.
Divide by 80.
Take logarithms on both sides.

35. Solution MODELING NEWTON’S LAW OF COOLING Example 6
Now use the remaining pair of values in this equation to find k.
In ex = x
Thus,

36. Find the temperature after half an hour. Solution
MODELING NEWTON’S LAW OF COOLING
Example 6 b.
Find the temperature after half an hour.
Solution
To find the temperature after ½ hr, let t = ½ in the model from part (a).
Model from part (a)
Let t = ½ .

37. c. Solution MODELING NEWTON’S LAW OF COOLING Example 6
How long will it take for the coffee to cool to 50°C?
Solution
To find how long it will take for the coffee to cool to 50°C, let (t) = 50.
Let (t) = 50.
Subtract 20.
Divide by 80.

38. c. Solution MODELING NEWTON’S LAW OF COOLING Example 6
How long will it take for the coffee to cool to 50°C?
Solution
To find how long it will take for the coffee to cool to 50°C, let (t) = 50.
Take logarithms on both sides.
In ex = x