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 revisit stage 2 of the What's Behind My Back center, which was first introduced in grade 1. Students begin with a tower of 10 connecting cubes. They break apart the tower and represent the two parts with a drawing and an expression. Throughout the activity, students practice using what they know about sums of 10 and the relationship between addition and subtraction to find unknown addends. Students explain their reasoning about how they know how many cubes are missing to their partner (MP3). As students discuss, listen for the ways students show how they are looking for and using the counting sequence, properties of operations, or the relationship between addition and subtraction (MP7).
MLR8 Discussion Supports.
Make connections between representations visible. Think aloud, use gestures and a labeled display to show connections between parts of the equation and the cubes that are visible and those that are hidden.
Advances: Listening, Representing
Required Materials
Materials to Gather
Connecting cubes
Materials to Copy
What's Behind My Back Stage 2 Recording Sheet Grade 1
Required Preparation
Gather a tower of 10 connecting cubes to use in the launch.
Launch
Groups of 2
Give each group 10 connecting cubes.
“We are going to play a game you may have played in first grade called What’s Behind My Back?”
Show students a tower of 10 cubes. Put the tower behind your back. Break off 4 cubes, and display the rest of the tower.
“How many cubes are behind my back? How do you know?” (4 because there are 6 and 4 more makes 10. I know because
\(10 - 4 = 6\)
.)
30 seconds: quiet think time
1 minute: partner discussion
Share responses.
“After one partner has hidden the cubes, switch roles.”
Activity
10 minutes: partner work time
Activity Synthesis
Ask a student to play a round of the game with the class.
As the student displays the tower, write the related addition equation with an unknown addend on the board. (for example, if a student shows 2 blocks, write
\(2+\underline{\hspace{.5 cm}}=10\)
.)
“How does this equation represent the game?”
“What number makes the equation true?”
“How can we represent the game with a subtraction equation?” (
\(10 - 2 = 8\)
or
\(10 - 8 = 2\)
)
If needed, ask students to show how the numbers in the addition and subtraction equations represent the cubes in the round.


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