Lesson 2
Ways to Look at Triangles
Lesson Purpose
Lesson Narrative
In the previous lesson, students sorted and categorized twodimensional shapes using any attribute of their choice. In this lesson, students narrow the set of shapes to triangles. They analyze and sort triangles based on their sides and angles.
Along the way, students begin to notice new attributes of triangles and make generalizations about them. They know that all triangles have three sides and three angles. Now they also see, for instance, that some triangles contain right angles, some triangles have equal sides and can be folded into two equal halves, triangles never seem to contain more than one obtuse angle, and so on. Students learn that right triangles are a subgroup of triangles and use what they know about identifying right angles and perpendicular sides to identify the properties of right triangles.
The process of sorting shapes and identifying attributes encourages students to look for structure (MP7), which students will use in later lessons to reason about symmetry and measurements in twodimensional figures.
 Action and Expression
 MLR8
Learning Goals
Teacher Facing
 Classify triangles based on the their side lengths and size of their angles.
Student Facing
 Let’s sort and analyze triangles.
Required Materials
Materials to Gather
Required Preparation
Activity 1:
 Each group needs a set of shape cards from the previous lesson. If time permits, separate the triangle cards from each set in advance.
 Gather the Collect and Display chart from the previous lesson for display in the activity synthesis.
CCSS Standards
Addressing
Lesson Timeline
Warmup  10 min 
Activity 1  25 min 
Activity 2  10 min 
Lesson Synthesis  10 min 
Cooldown  5 min 
Teacher Reflection Questions
Suggested Centers
 Picture Books (K–5), Stage 3: Find Shapes (Supporting)
Print Formatted Materials
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Additional Resources
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