Lesson 3

Making Scaled Copies

Lesson Narrative

In the previous lesson, students learned that we can use scale factors to describe the relationship between corresponding lengths in scaled figures. Here they apply this idea to draw scaled copies of simple shapes on and off a grid. They also strengthen their understanding that the relationship between scaled copies is multiplicative, not additive. Students make careful arguments about the scaling process (MP3), have opportunities to use tools like tracing paper or index cards strategically (MP5).

As students draw scaled copies and analyze scaled relationships more closely, encourage them to continue using the terms scale factor and corresponding in their reasoning.

Learning Goals

Teacher Facing

• Critique (orally and in writing) different strategies (expressed in words and through other representations) for creating scaled copies of a figure.
• Draw a scaled copy of a given figure using a given scale factor.
• Generalize (orally and in writing) that the relationship between the side lengths of a figure and its scaled copy is multiplicative, not additive.

Student Facing

Let’s draw scaled copies.

Required Preparation

Make sure students have access to their geometry toolkits, especially tracing paper and index cards.

Student Facing

• I can draw a scaled copy of a figure using a given scale factor.
• I know what operation to use on the side lengths of a figure to produce a scaled copy.

Building On

Building Towards

Glossary Entries

• corresponding

When part of an original figure matches up with part of a copy, we call them corresponding parts. These could be points, segments, angles, or distances.

For example, point $$B$$ in the first triangle corresponds to point $$E$$ in the second triangle. Segment $$AC$$ corresponds to segment $$DF$$.

• scale factor

To create a scaled copy, we multiply all the lengths in the original figure by the same number. This number is called the scale factor.

In this example, the scale factor is 1.5, because $$4 \boldcdot (1.5) = 6$$, $$5 \boldcdot (1.5)=7.5$$, and $$6 \boldcdot (1.5)=9$$.

• scaled copy

A scaled copy is a copy of an figure where every length in the original figure is multiplied by the same number.

For example, triangle $$DEF$$ is a scaled copy of triangle $$ABC$$. Each side length on triangle $$ABC$$ was multiplied by 1.5 to get the corresponding side length on triangle $$DEF$$.