Lesson 2

The purpose of this lesson is for students to classify triangles based on the their side lengths and the size of their angles. Students identify triangles with a right angle as right triangles.

In the previous lesson, students sorted and categorized two-dimensional 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 sub-group 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 two-dimensional figures.

• 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.

What question went unanswered in this lesson? How will you revisit the ideas underlying this question in the lessons that follow?