standard_description: Multiply a vector (regarded as a matrix with one column) by a matrix of suitable dimensions to produce another vector. Work with matrices as transformations of vectors.
problem_activity: Narrative
The purpose of this activity is for students to find the area of rectangles by counting squares. Larger rectangles provide more opportunities for students to practice counting strategies using the structure of the rectangles to group the individual squares (MP7). Rectangles in this activity lend themselves to show groups of twos, fives, or tens in rows or columns. Students may also see other ways to create equal groups within rectangles. For example, the second rectangle with an area of 30 square units can be seen as 3 groups of ten. If students finish quickly, encourage them to confirm the area by counting another way. Emphasize that each area is in square units.
MLR8 Discussion Supports.
Synthesis: For each observation that is shared, invite students to turn to a partner and restate what they heard, using precise mathematical language, such as area or square units.
Advances: Listening, Speaking
Launch
Groups of 2
Activity
“Find the area of each rectangle. Be ready to explain your reasoning.”
5–7 minutes: independent work time
Monitor for counting strategies such as:
Counting the squares in a row or a column and then skip-counting by that number for subsequent rows or columns.
Grouping by twos, fives, tens, or other numbers.
“Now share your strategies with your partner.”
2–3 minutes: partner discussion
Student Facing
Find the area of each rectangle and include the units. Explain or show your reasoning.
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Student Response
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Advancing Student Thinking
If students miscount the square units in the rectangles, consider asking:
“Tell me how you counted the squares.”
“How could you keep track of your count as you count the squares?”
Activity Synthesis
Invite 3–4 previously selected students to share how they found the area of one of the rectangles. Try to highlight a variety of strategies.
Consider asking, “Did your strategy change from rectangle to rectangle?”


aligns: no