Lesson plan

4. Identify similar shapes by applying scale factors (A)

teaches Common Core State Standards CCSS.Math.Content.8.G.A.3 http://corestandards.org/Math/Content/8/G/A/3
teaches Common Core State Standards CCSS.Math.Practice.MP3 http://corestandards.org/Math/Practice/MP3
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Lesson objective:  Identify similarity in shapes with the same name.

This lesson helps to build fluency with identifying attributes of similar shapes. A coordinate plane is used here because it allows students to determine lengths either by the numbers on the axes or by squares on the plane. This work develops students' understanding that a dilation of a two-dimensional figure produces a figure that is smaller than, larger than, or the same size as the original figure.

Students engage in Mathematical Practice 3 (Construct viable arguments and critique the reasoning of others) as they discuss the relationships formed by similar shapes.  Students should notice that the angles remain the same while the sides may decrease, increase, or remain the same.  Similar figures have equal ratios describing the relationship of lengths.

Key vocabulary:

  • ratio
  • scale factor
  • similar
  • transformation

Special materials needed:

  • coordinate planes
  • highlighters or colored pencils to help students color code when necessary