Unit 2, Lesson 8
Similar Triangles
Grade 8
Preparation
Lesson Narrative
The purpose of this lesson is for students to see that if triangles have two corresponding angles with equal measure, then they are similar. Since the sum of angle measures in a triangle is always 180 degrees, having two angle measures will determine the third, making two angles all that is necessary to show similarity.
Students begin by observing features of a complex figure and crafting questions that can be asked about the figure. Then, by noticing patterns in triangles created out of dried pasta with given angle constraints, they deduce the criteria for similarity in terms of angle measures (MP7). An optional activity provides additional practice for identifying and justifying that two triangles are similar, using the same figure seen in the Warm-up.
- Generalize a process for identifying similar triangles and justify (orally) that finding two pairs of congruent angles is sufficient to show similarity.
- Justify (orally) that two triangles are similar by finding a sequence of transformations that takes one triangle to the other or checking that two pairs of corresponding angles are congruent.
Let’s look at similar triangles.
Required Materials
Activity 3
Required Preparation
Provide access to geometry toolkits, including blank paper, tape, a ruler, and a protractor. For the dried pasta that will be used to create the sides of the triangles, we recommend fettuccine or linguine so it doesn’t roll off the table and is easy to break as needed.
Provide access to geometry toolkits.