Lesson 2Scale Factors and Making Scaled Copies

Warm Up: Number Talk: Multiplying by a Unit Fraction

Activity 1: Scaled Triangles

Problem 1

Here is Triangle , followed by a number of other triangles.

Your teacher will assign you two of the triangles to look at.

1. For each of your assigned triangles, is it a scaled copy of Triangle ? Be prepared to explain your reasoning.

2. As a group, identify all the scaled copies of Triangle in the collection. Discuss your thinking. If you disagree, work to reach an agreement.

3. List all the triangles that are scaled copies in the table.

   Record the side lengths that correspond to the side lengths of Triangle listed in each column.

   Triangle	3	4	5

4. Explain or show how each copy has been scaled from the original (Triangle ).

Are you ready for more?

Activity 2: Which Operations? (Part 1)

Problem 1

Diego and Jada want to scale this polygon so the side that corresponds to 15 units in the original is 5 units in the scaled copy.

Diego and Jada each use a different operation to find the new side lengths. Here are their finished drawings.

1. What operation do you think Diego used to calculate the lengths for his drawing?

2. What operation do you think Jada used to calculate the lengths for her drawing?

3. Did each method produce a scaled copy of the polygon? Explain your reasoning.

Activity 3: Which Operations? (Part 2)

Problem 1

Andre wants to make a scaled copy of Jada’s drawing so the side that corresponds to 4 units in Jada’s polygon is 8 units in his scaled copy.

1. Andre says “I wonder if I should add 4 units to the lengths of all of the segments?” What would you say in response to Andre? Explain or show your reasoning.

2. Create the scaled copy that Andre wants. If you get stuck, consider using the edge of an index card or paper to measure the lengths needed to draw the copy.

Are you ready for more?

Lesson Summary

Here are two polygons. Polygon 2 is a scaled copy of Polygon 1.

The scale factor between Polygon 1 and Polygon 2 is 2, because all of the lengths in Polygon 2 are 2 times the corresponding lengths in Polygon 1. The angle measures in Polygon 2 are the same as the corresponding angle measures in Polygon 1. For example, the measure of angle is the same as the measure of angle

Creating a scaled copy involves multiplying the lengths in the original figure by a scale factor.

For example, to make a scaled copy of triangle where the base is 8 units, we would use a scale factor of 4. This means multiplying all the side lengths by 4, so in triangle , each side is 4 times as long as the corresponding side in triangle .