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: Narrative
The purpose of this activity is for students to apply what they have learned in this section to compare a number to the product of that number with a fraction. There are two cases where the fraction has a value of 1. Students may identify that the value of the fraction is 1 and use what they know about multiplying a number by 1. They may also use their knowledge of how to multiply fractions and the calculations for these products have been made simpler so that students can find the product to make the comparison. For the other problems, the numbers are sufficiently complex that the most efficient way to compare is to think about the size of the factors.
MLR1 Stronger and Clearer Each Time.
Synthesis: Before the whole-class discussion, give students time to meet with 2–3 partners to share and get feedback on their response to whether the expressions are
\(<\)
,
\(>\)
, or
\(=\)
. Invite listeners to ask questions, to press for details and to suggest mathematical language. Give students 2–3 minutes to revise their written explanation based on the feedback they receive.
Advances: Writing, Speaking, Listening
Launch
Groups of 2
Activity
1–2 minutes: quiet think time
10–12 minutes: partner work time
For the expressions
\(\frac{1}{4} \times \frac{5}{5}\)
and
\(\frac{10}{10} \times \frac{1}{2}\)
monitor for students who calculate the products and for students who reason about the size of the factors.
Student Facing
Write
\(<\)
,
\(>\)
, or
\(=\)
in each blank to make true statements.
Choose one problem and explain or show your reasoning.
\(567 \, \underline{\hspace{0.7cm}} \,345 \times 567\)
\(\frac{4}{5} \times 851 \, \underline{\hspace{0.7cm}} \, 851\)
\(\frac{1}{4} \, \underline{\hspace{0.7cm}} \, \frac{5}{5} \times \frac{1}{4}\)
\(\frac{103}{104} \, \underline{\hspace{0.7cm}} \, \frac{103}{104} \times \frac{103}{104}\)
\(\frac{99}{8} \times \frac{23}{22} \, \underline{\hspace{0.7cm}} \, \frac{99}{8}\)
\(\frac{10}{10} \times \frac{1}{2} \, \underline{\hspace{0.7cm}} \, \frac{1}{2}\)
\(\frac{100}{7} \times \frac{9}{13} \, \underline{\hspace{0.7cm}} \, \frac{9}{13}\)
Student Response
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Activity Synthesis
Invite students to share their solution for comparing
\(\frac{10}{10} \times \frac{1}{2}\)
and
\(\frac{1}{2}\)
.
“How do you know that
\(\frac{10}{20} = \frac{1}{2}\)
?” (If you cut the half into 10 equal pieces then you get 10 pieces and there are 20 in the whole.)
“Can you use the equation
\(\frac{10}{10} = 1\)
to see that
\(\frac{10}{10} \times \frac{1}{2} = \frac{1}{2}\)
?” (Yes, because 1 times any number is that same number.)
Invite students to share their solution for comparing
\(\frac{103}{104}\)
and
\(\frac{103}{104} \times \frac{103}{104}\)
.
“Did you find the product to compare?” (No, the numbers are big so it would have taken a long time.)
“What did you do to compare the numbers?” (I know that
\(\frac{103}{104}\)
is less than 1, it’s
\(1 - \frac{1}{104}\)
. So when I multiply it by any number I get less than that number.)


aligns: no