standard_description: Use similar triangles to explain why the slope m is the same between any two distinct points on a non-vertical line in the coordinate plane; derive the equation y = mx for a line through the origin and the equation y = mx + b for a line intercepting the vertical axis at b.
problem_activity: Optional activity
In this activity, students transfer what they learned with the pattern blocks to calculate the area of other scaled shapes (MP8). In groups of 2, students draw scaled copies of either a parallelogram or a triangle and calculate the areas. Then, each group compares their results with those of a group that worked on the other shape. They find that the scaled areas of two shapes are the same (even though the starting shapes are different and have different measurements) and attribute this to the fact that the two shapes had the same original area and were scaled using the same scale factors.
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While students are not asked to reason about scaled areas by tiling (as they had done in the previous activities), each scaled copy can be tiled to illustrate how length measurements have scaled and how the original area has changed. Some students may choose to draw scaled copies and think about scaled areas this way.
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As students find the areas of copies with scale factors 5 and
\(\frac35\)
without drawing (for the last question), monitor for these methods, depending on their understanding of or comfort with the
\(\text{(scale factor)}^2\)
pattern:
Scaling the original base and height and then multiplying to find the area
Multiplying the original area by the square of the scale factor
Select students using each approach. Invite them to share their reasoning, sequenced in this order, during the discussion.
You will need the Area of Scaled Parallelograms and Triangles blackline master for this activity.
Launch
Arrange students in groups of 2. Provide access to geometry toolkits.
Distribute slips showing the parallelogram to half the groups and the triangle to the others. Give students 1 minute of quiet work time for the first question, and then time to complete the rest of the task with their partner.
Engagement: Develop Effort and Persistence.
Connect a new concept to one with which students have experienced success. For example, draw students attention to the warm-up and remind them how to build scaled copies of the rhombus with scale factors of 2, 3, and 4. Ask students how they can use this technique to draw scaled copies of the parallelogram or triangle with scale factors of 2, 3, and 5.
Supports accessibility for: Social-emotional skills; Conceptual processing
Student Facing
Your teacher will give you a figure with measurements in centimeters. What is the
area
of your figure? How do you know?
Work with your partner to draw scaled copies of your figure, using each scale factor in the table. Complete the table with the measurements of your scaled copies.
|  scale factor   | base (cm) | height (cm) | area (cm                       2                      ) |
|-----------------|-----------|-------------|---------------------------------------------------------|
|        1        |           |             |                                                         |
|        2        |           |             |                                                         |
|        3        |           |             |                                                         |
| \(\frac{1}{2}\) |           |             |                                                         |
| \(\frac{1}{3}\) |           |             |                                                         |
Compare your results with a group that worked with a different figure. What is the same about your answers? What is different?
If you drew scaled copies of your figure with the following scale factors, what would their areas be? Discuss your thinking. If you disagree, work to reach an agreement. Be prepared to explain your reasoning.
|  scale factor   | area (cm                       2                      ) |
|-----------------|---------------------------------------------------------|
|        5        |                                                         |
| \(\frac{3}{5}\) |                                                         |
Student Response
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Anticipated Misconceptions
Students may not remember how to calculate the area of parallelograms and triangles. Make sure that they have the correct area of 10 square units for their original shape before they calculate the area of their scaled copies.
When drawing their scaled copies, some students might not focus on making corresponding angles equal. As long as they scale the base and height of their polygon correctly, this will not impact their area calculations. If time permits, however, prompt them to check their angles using tracing paper or a protractor.
Some students might focus unnecessarily on measuring other side lengths of their polygon, instead of attending only to base and height. If time is limited, encourage them to scale the base and height carefully and check or measure the angles instead.
Activity Synthesis
Invite selected students to share their solutions. Then focus class discussion on two themes: how the values in the tables for the two shapes compare, and how students determined the scaled areas for the scale factors 5 and
\(\frac35\)
. Ask questions such as:
What did you notice when you compared your answerswithanother group that worked with the other figure? (When the scale factors are the same, the scaled areas are the same, though the bases and heights are different.)
How did you find the scaled areas for scale factors of 5 and
\(\frac35\)
? (By scaling the original base and height and multiplying the scaled measurements; by multiplying the original area by
\(\text{(scale factor)}^2\)
.)
How is the process for finding scaled area here the same as and different than that in the previous activities with pattern blocks? (The area units are different; the pattern of squaring the scale factor is the same.)
Highlight the connection between the two ways of finding scaled areas. Point out that when we multiply the base and height each by the scale factor and then multiply the results, we are essentially multiplying the original lengths by the scale factor two times. The effect of this process is the same as multiplying the original area by
\(\text{(scale factor)}^2\)
.
Representing, Speaking, and Listening: MLR7 Compare and Connect.
Invite students to prepare a display that shows their approach to finding the areas for scale factors of 5 and
\(\frac35\)
. Ask students to research how other students approached the problem, in search of a method that is different from their own. Challenge students to describe why the different approaches result in the same answers. During the whole-class discussion, emphasize the language used to explain the different strategies, especially phrases related to “squaring” and “multiplying a number by itself.” This will strengthen students' mathematical language use and reasoning based on the relationship between scale factors and area.
Design Principle(s): Maximize meta-awareness


aligns: no