standard_description: Identify when two expressions are equivalent (i.e., when the two expressions name the same number regardless of which value is substituted into them). For example, the expressions y + y + y and 3y are equivalent because they name the same number regardless of which number y stands for.
problem_activity: Narrative
The purpose of this activity is for students to find the volume of a figure in different ways. The given figure can be decomposed in two ways into rectangular prisms by making different cuts. However, it can also be found using a single, larger rectangular prism by removing a smaller rectangular prism. This provides an opportunity to express its volume as a difference of volumes of rectangular prisms. Students may notice this feature, and it is highlighted in the activity synthesis.
When students decide whether or not they have the same expressions, they need to reason carefully about what “the same” means. They consider if the order of the factors is different, is it the same expression and if the order of the addends is different, is it the same expression. Students use what they know about volume, geometric figures, and the properties of operations to justify the equivalence of the expressions and critique their peers' reasoning (MP2, MP3, MP7).
MLR8 Discussion Supports.
Display sentence frames to support partner discussion: “First, I _____ because . . . .” , “_____ and _____ are the same because . . .”, “_____ and _____ are different because . . .”
Advances: Conversing, Representing
Launch
Groups of 2
“You are going to look for different expressions to calculate the volume of a figure.”
Activity
5 minutes: partner work time
Monitor for students who draw a vertical line to show where they decompose the figure to share during the synthesis.
Student Facing
[IMG]
Write an expression to represent the volume of the figure in unit cubes.
Compare expressions with your partner.
How are they the same?
How are they different?
If they are the same, try to find another way to represent the volume.
Student Response
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Advancing Student Thinking
If students did not see the figure as a cube missing a rectangular prism, ask them to build the figure and the missing piece that can be added to make it a cube.
Activity Synthesis
Invite students to share different expressions for the volume of the figure.
Display expression:
\((3 \times 5 \times 5) + (2 \times 2 \times 5)\)
“How does the expression represent the volume of the figure?” (
\(3 \times 5 \times 5\)
represents the prism that is 3 by 5 by 5 cubes tall and the
\(2 \times 2 \times 5\)
represents the prism that is 2 by 2 by 5 cubes tall.)
If no student wrote the expression
\((5 \times 5 \times 5) – (2 \times 3 \times 5)\)
, display this expression.
“How does the expression show the volume of the figure in cubic units?” (There is a 5 by 5 by 5 cube and a piece has been taken away. The piece that is taken away measures 2 cubes by 3 cubes by 5 cubes.)


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