Lesson plan

5. Identify similarity in triangles by congruent corresponding angles (C)

teaches Common Core State Standards CCSS.Math.Content.8.G.A.5 http://corestandards.org/Math/Content/8/G/A/5
teaches Common Core State Standards CCSS.Math.Practice.MP6 http://corestandards.org/Math/Practice/MP6
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Lesson objective: Understand that similarity between shapes can be proven by identifying two pairs of congruent corresponding angles.

Students bring prior knowledge of the attributes of dilated shapes from 8.G.A.3. This prior knowledge is extended to proving shapes are similar as students identify two pairs of congruent corresponding angles. A conceptual challenge students may encounter is identifying two pairs of congruent corresponding angles. 

The concept is developed through work with birdhouses, which helps students understand that one pair of congruent corresponding angles is not sufficient to determine similarity.

This work helps students deepen their understanding of equivalence because students will identify the measurements corresponding angles.

Students engage in Mathematical Practice 6 (attend to precision) as they find the missing measurements of angles. Students should not assume angles are congruent without determining the measurements.

Key vocabulary:

  • corresponding angles
  • dilation
  • equilateral triangle
  • isosceles triangle
  • similar

Special materials needed:

  • protractors