standard_description: Derive the formula for the sum of a finite geometric series (when the common ratio is not 1), and use the formula to solve problems. For example, calculate mortgage payments.
problem_activity: Activity
In this activity, students practice constructing an exponential function and evaluating it at a fractional input value as they translate between decades and years. They also think about how to express a percent increase over a fractional interval of input—specifically, how to express the annual growth rate of a population given the growth rate for one decade. They analyze and articulate why the growth rate for one year is
not
one tenth of that of one decade (MP3).
Launch
Tell students that, as of 2017, Nigeria is the seventh-most populous country in the world. It has grown at a fairly steady (and relatively high) rate since 1980.
Speaking, Reading: MLR5 Co-Craft Questions.
Use this routine to orient students to the language of exponential functions in context. Ask students to keep their books or devices closed, and display only the image of the map and the problem statement, without revealing the questions that follow. Arrange students in groups of 2, and invite them to create a list of mathematical questions that could be asked about the situation.  Listen for and amplify questions about growth and decay factors, percent increase or decrease, fractional intervals, and comparisons of growth rates. Invite 2–3 groups to share their questions with the class.
Design Principle(s): Maximize meta-awareness; Support sense-making
Student Facing
[IMG]
In 1990, Nigeria had a population of about 95.3 million. By 2000, there were about 122.4 million people, an increase of about 28.4%. During that decade, the population can be reasonably modeled by an exponential function.
Express the population of Nigeria
\(f(d)\)
, in millions of people,
\(d\)
decades since 1990.
Write an expression to represent the population of Nigeria in 1996.
A student said, “The population of Nigeria grew at a rate of 2.84% every year.”
Explain or show why the student’s statement is incorrect.
Find the correct annual growth rate. Explain or show your reasoning.
Student Response
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Anticipated Misconceptions
If students struggle to identify the input to their function where
\(d\)
is not an integer, remind them that there are 10 years in a decade. Then ask them how many decades is 6 years? What about one year?
Activity Synthesis
Focus the discussion on why the annual growth rate is not
\(\frac1{10}\)
of 28.4%. Consider asking students:
“Why might someone think the annual population growth rate is 2.84%?” (Because 28.4 divided by 10 is 2.84, and they assume that population was growing linearly, or by the same amount each year, rather than by the same factor each year.)
“How can we determine the growth factor for one year?” (We know that when the annual growth factor, say
\(b\)
, is raised to an exponent of 10, it must equal 1.284, or
\(b^{10} = 1.284\)
, so
\(b\)
must be
\(1.284^\frac{1}{10}\)
or
\(\sqrt[10]{1.284}\)
because raising that number to an exponent of 10 gives 1.284.)
“How do we express the factor as a growth rate? (
\(1.284^\frac{1}{10} \approx 1.0258\)
, so the growth rate is 2.58%.)
Representation: Internalize Comprehension.
Use color and annotations to illustrate student thinking. As students share their reasoning about the last question, scribe their thinking on a visible display. To illustrate why the growth factor for one year is not
\(\frac{1}{10}\)
of that of a decade, it may be helpful to return to a diagram that is configured like a double number line (especially if used in the last lesson). Use the double number line of years and decades to annotate and show the growth factor. When students explain that the growth factor for one decade is 1.284, draw an arrow across the decade line and label it
\(\boldcdot 1.284\)
. Then draw small arrows for the years labeled with
\(\boldcdot b\)
for each segment. Label the decade line with the corresponding equation. Encourage students to refer to the diagram when explaining why
\(b\)
is not one-tenth the growth rate of a decade. When students first explain the use of the fractional exponent to calculate the yearly population growth, label the line representing years with the corresponding equation.
Supports accessibility for: Visual-spatial processing; Conceptual processing


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