standard_description: Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.
problem_activity: Narrative
The purpose of this activity is for students to begin to distinguish rectangles from other shapes. By working with variants and non-examples of rectangles, students begin to develop their understanding of what makes a shape a rectangle. When they discuss which group a shape should be placed in, students informally describe the attributes of the shape.
MLR8 Discussion Supports.
Students should take turns finding a match and explaining their reasoning to their partner. Display the following sentence frames for all to see: “I noticed _____, so I matched . . . .” Encourage students to challenge each other when they disagree.
Advances Conversing, Listening
Required Materials
Materials to Copy
Rectangle Sort Cards
Required Preparation
Cut out rectangle cards from the blackline master. Each group of 4 needs 1 set of cards.
Launch
Groups of 4
Give each group a set of cards.
Draw or display a rectangle:
[IMG]
“Tell your partner two things you know about this shape.” (There are 4 sides. It looks like a door. It is a rectangle. It is long and flat.)
30 seconds: quiet think time
30 seconds: partner discussion
Share and record responses.
If needed, ask “What is the name of this shape?”
30 seconds: quiet think time
“This shape is a
rectangle
.”
Activity
“Work with your group to sort the shapes into two groups. Put the shapes that are rectangles on the left side of your page. Put the shapes that are not rectangles on the right side of your page. When you place a shape, tell your group why you think the shape belongs in that group.”
4 minutes: small-group work time
“Write a number to show how many shapes are in each group.”
1 minute: independent work time
“Which group has fewer shapes? How do you know?”
30 seconds: quiet think time
30 seconds: partner discussion
Student Facing
Let’s put the shapes into 2 groups.
Rectangle
[IMG]
[IMG]
[IMG]
Not a Rectangle
[IMG]
[IMG]
Student Response
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Advancing Student Thinking
If students put shapes other than rectangles into the group of rectangles, consider asking:
“How did you choose which group to put this shape in?”
“What is alike, or the same, about all of the shapes in this group?”
Activity Synthesis
Invite 1–2 groups to share how they sorted the shapes.
Draw or display a square:
[IMG]
“Which group did you put this shape in?” (We put it with the rectangles because it has 4 sides. We didn’t put it with the rectangles because it is a square.)
“This shape is a
square
, which is a special rectangle because all four sides are the same length.”


aligns: no