1. Exponential Growth and Decay
Section 3.5

2. Objectives
Solve word problems requiring exponential models.

3. Find the time required for an investment of $5000 to grow to $6800 at an interest rate of 7.5% compounded quarterly.
Formula needed:

4. Find the time required for an investment of $5000 to grow to $6800 at an interest rate of 7.5% compounded quarterly.
Change to exponential form
Take the logarithm of both sides
Change of base
It will take approximately years.

5. The population of a certain city was in 1998, and the observed relative growth rate is 2% per year.
Find a function that models the population after t years.
Find the projected population in the year 2004.
In what year will the population reach ?

6. The population of a certain city was in 1998, and the observed relative growth rate is 2% per year.
Find a function that models the population after t years.
Populations are assumed to have continuous growth. Thus we use the formula
We are going to let t = 0 represent the year 1998 and t is the number of years since 1998.
This model will give us the population t years after 1998.

7. The population of a certain city was in 1998, and the observed relative growth rate is 2% per year.
Find the projected population in the year 2004.
We use the model from the first part of the question
Now we plug in for t. We need to know what t will represent the year We find that using a subtraction problem = 6 tells us that 2004 is 6 years since 1998 and thus our t is 6.
This is not the answer. People must be whole, thus we must round this number. Either people or people would be considered correct.

8. The population of a certain city was in 1998, and the observed relative growth rate is 2% per year.
In what year will the population reach ?
We use the model from the first part of the question
Now we plug in for A and solve for t.
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9. The population of a certain city was in 1998, and the observed relative growth rate is 2% per year.
In what year will the population reach ?
take a logarithm of both sides of the equation
change to logarithmic form OR
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10. The population of a certain city was in 1998, and the observed relative growth rate is 2% per year.
In what year will the population reach ?
We still don’t have the answer to the question. We know that years after 1998 the population will reach We need to know what year that is. We find this by adding =
Now do we round up to 2010 or keep it at We must always keep it at the lower year (no matter what the decimal). Any decimal part of a year is still during that year. That the answer to the question is in the year 2009, the population will reach

11. Find the relative growth rate.
The count in a bacteria culture was 600 after 15 minutes and after 35 minutes.  Assume that growth can be modeled exponentially by a function of the form where t is in minutes.
Find the relative growth rate.
We do not know a starting bacteria count, but the growth rate can be figured from any point in time to any other point in time as long as we use the passage of time as the t value.
We will use 600 as the starting population only for the purpose of this part of the problem. We then have bacteria 20 minutes later (the amount of time between 15 and 35 minutes). With these numbers we can solve for r.
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12. Find the relative growth rate.
The count in a bacteria culture was 600 after 15 minutes and after 35 minutes.  Assume that growth can be modeled exponentially by a function of the form where t is in minutes.
Find the relative growth rate.
We can now use this r for the other parts of this problem. It will be important tat we do not round our r value in future parts of the problem.

13. What was the initial size of the culture?
The count in a bacteria culture was 600 after 15 minutes and after 35 minutes.  Assume that growth can be modeled exponentially by a function of the form where t is in minutes.
What was the initial size of the culture?
We know the relative growth rate r. We also have two different populations after two different periods of time. We can use either one for our population A after time t. We will need to solve for P.
I will use (15, 600) as the population after time t.
Since bacteria must be whole, the initial population must be 51 bacteria. This means that out model for future parts of the problem will be

14. Find the doubling period in minutes.
The count in a bacteria culture was 600 after 15 minutes and after 35 minutes.  Assume that growth can be modeled exponentially by a function of the form where t is in minutes.
Find the doubling period in minutes.
Doubling time is how long it takes for the number of bacteria to get to be two times as many as the initial population. This means that we need to plug in an ending population of 2*51 or 102.
The doubling time is approximately minutes.

15. Find the population after 110 minutes.
The count in a bacteria culture was 600 after 15 minutes and after 35 minutes.  Assume that growth can be modeled exponentially by a function of the form where t is in minutes.
Find the population after 110 minutes.
For this one we just plug 110 into the model for t.

16. When will the population reach 15000?
The count in a bacteria culture was 600 after 15 minutes and after 35 minutes.  Assume that growth can be modeled exponentially by a function of the form where t is in minutes.
When will the population reach 15000?
For this one, we plug in for A(t) and solve the t.
The population will reach in approximately minutes.

17. Find a function that models the mass m(t) remaining after t years.
The half-life of strontium-90 is 28 years.  Suppose we have a 80 mg sample.
Find a function that models the mass m(t) remaining after t years.
In order to make a model, we need an initial amount and a decay rate. We are given the initial amount is 80 mg. We will have to calculate r. In order to do this, we will use the definition of half-life. Half-life is the number of years it takes to get from one amount to half of that amount. Thus for our problem, we know that in 28 years we will have 40 mg (half of 80). Now we plug all this into the continuous decay formula (same as continuous growth formula).
Model:
It will be important tat we do not round our r value in future parts of the problem.

18. How much of the sample will remain after 100 years?
The half-life of strontium-90 is 28 years.  Suppose we have a 80 mg sample.
How much of the sample will remain after 100 years?
Model:
We will use the model found in the previous part of the problem and plug in 100 for the t.
In 100 years, there will be grams

19. How long will it take the sample to decay to a mass of 20 mg?
The half-life of strontium-90 is 28 years.  Suppose we have a 80 mg sample.
How long will it take the sample to decay to a mass of 20 mg?
Model:
For this part of the problem, we will use the model found in part 1. We will plug in 20 for A and solve for t.
It will take 56 years to get down to 20 mg. You could also have done this just using the definition of half-life.

20. A wooden artifact from an ancient tomb contains 35% of the carbon-14 that is present in living trees.  How long ago was the artifact made?  (The half-life of carbon-14 is 5730 years.)
For this problem we will need to find a model to represent the decay of carbon-14. This will require that we find r. Once we have r and the model, we will be able to solve the problem.
It is assumed that a living tree has 100% of the carbon-14 in living plants. Once cut down, the amount of carbon-14 starts decreasing (decaying) based on the half-life. This means that we can use the half-life information with the staring point The half-life information tells us that we have 50% of the carbon-14 after 5730 years.
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21. A wooden artifact from an ancient tomb contains 35% of the carbon-14 that is present in living trees.  How long ago was the artifact made?  (The half-life of carbon-14 is 5730 years.)
Finding r using the half-life information
This means that our model is
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22. A wooden artifact from an ancient tomb contains 35% of the carbon-14 that is present in living trees.  How long ago was the artifact made?  (The half-life of carbon-14 is 5730 years.)
We use the model from the previous page. Since the ending amount of carbon-14 is 35%, we will plug 35 in for A(t) – the amount after time t. Now we solve for t.
This means that our wooden artifact is approximately years old.

23. An infectious strain of bacteria increases in number at a relative growth rate of 190% per hour.  When a certain critical number of bacteria are present in the bloodstream, a person becomes ill.  If a single bacterium infects a person, the critical level is reached in 24 hours.  How long will it take for the critical level to be reached if the same person is infected with 10 bacteria?
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24. An infectious strain of bacteria increases in number at a relative growth rate of 190% per hour.  When a certain critical number of bacteria are present in the bloodstream, a person becomes ill.  If a single bacterium infects a person, the critical level is reached in 24 hours.  How long will it take for the critical level to be reached if the same person is infected with 10 bacteria?
In order to answer this question, we have to know the relative growth rate and the number of bacteria that is classified as the critical level. In the problem, we are give the relative growth rate (r = 1.9). We know that if you start with 1 bacteria, you will have the critical level number in 24 hours. Thus we can use a model to find the critical level which we need to answer the question.
Thus we need to find out how long it will take to reach this critical number We will plug in this number for A and 10 for P and solve for t
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25. An infectious strain of bacteria increases in number at a relative growth rate of 190% per hour.  When a certain critical number of bacteria are present in the bloodstream, a person becomes ill.  If a single bacterium infects a person, the critical level is reached in 24 hours.  How long will it take for the critical level to be reached if the same person is infected with 10 bacteria?
Thus we need to find out how long it will take to reach this critical number We will plug in this number for A We will plug in 10 for P since we are asked how long it takes to get to the critical level if we start with 10 bacteria. Now we solve for t.
It will take hours to reach the critical level or for a person to become ill.