APPLICATIONS OF EXPONENTIAL AND LOGARITHMIC FUNCTIONS
(Population Word Problems)
To solve an exponential or logarithmic word problem, convert the narrative to an equation and solve the equation.
Now we are going to get serious.
Example 19: According to the U.S. Bureau of the Census, in the year 1850 the population of the United States was 23,197,876; in 1900, the population was 62,947,714.
(A) Assuming that the population grew exponentially during this
period, find the model (equation) that describes the population
growth. What is the relative growth rate?
(B) Assuming continued growth at the same rate, predict the
1950 population.
(C) The actual population for 1950, according to the Bureau
of the Census, was 150,697,361. How does this compare to your
prediction in part (B)? Was the actual growth over the period
1900-1950 faster or slower than the exponential growth with the
growth rate in part (A)?
Solution and Explanations:
The problem states that the growth rate was exponential during the years 1850 through 1900. You many use any positive number for the base in the exponential equation (model). However, for the sake of standardization, let's choose the base e. The generic equation is
where a is the population when t = 0 and b is the relative growth rate with respect to the base e. Since the study started in 1850, we will let t = 0 in the year 1850.
rounded to .02. The equation is
The relative growth rate (with respect to the base e) between the years 1850 and 1900 was approximately 2%.
Predict the 1950 population using the model .
The actual population was 150,697,361 in 1950. Was the actual growth rate between 1900 and 1950 faster or slower than 2%?
rounded to .018.
The equation is
The relative growth rate (with respect to the base e) between the years 1900 and 1950 was approximately 1.8%, a little less than the relative growth rate of 2% between the years 1850 and 1900.
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Author: Nancy Marcus