# Lesson 14

Find the Area of Figures with Missing Sides

## Warm-up: Notice and Wonder: Mystery Sides (10 minutes)

### Narrative

The purpose of this warm-up is to elicit the idea that we can find the area of figures even though not all side lengths are given, which will be useful when students find missing side lengths and areas in a later activity. While students may notice and wonder many things about this figure, the missing side lengths are the important discussion points.

### Launch

- Groups of 2
- Display the image.
- “What do you notice? What do you wonder?”
- 1 minute: quiet think time

### Activity

- “Discuss your thinking with your partner.”
- 1 minute: partner discussion
- Share and record responses.

### Student Facing

What do you notice? What do you wonder?

### Student Response

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### Activity Synthesis

- “Do you think we could still find the area of this figure even though some of the side lengths are missing? Why or why not?” (Yes, we could use the side lengths we know to find the missing side lengths.)
- “How would you find the missing side lengths?” (I know the top side is 10 cm, and that is the horizontal length of the entire figure. If one part of that length is 4 cm, then the missing length is 6 cm. I know the vertical length of the figure is 9 cm. If one part of it is 2 cm, then the missing length is 7 cm.)

## Activity 1: The Mystery Side (10 minutes)

### Narrative

The purpose of this activity is for students to consider a strategy to find a missing side length of a figure composed of rectangles. They find the missing side length by decomposing the figure into non-overlapping rectangles.

### Launch

- Groups of 2
- Display the image.
- “Consider this figure. Take some time to think about Tyler’s reasoning for finding the missing side length.”
- 1–2 minutes: quiet think time

### Activity

- “Share your ideas about Tyler’s reasoning with your partner.”
- 2–3 minutes: partner discussion

### Student Facing

Tyler says that the missing side length is 5 meters because it looks longer than the sides that are 4 meters long.

Do you agree or disagree? Be prepared to explain your reasoning.

### Student Response

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### Activity Synthesis

- Invite students to share their ideas about Tyler’s reasoning.
- Consider asking:
- “What strategies did you or your partner use to find the missing side length?” (Encourage students to share on the displayed figure.)

## Activity 2: Practice with Mystery Sides (25 minutes)

### Narrative

The purpose of this activity is for students to find the area of a figure composed of rectangles with missing sides by decomposing it into two non-overlapping rectangles. There are several ways to approach these problems and students are given freedom to choose their own strategy to find the area. They share their reasoning with a student who has used a different method and prepare to share this new method during the class discussion (MP3). The synthesis should focus on ways to find missing side lengths and emphasize different ways to find the area of the figure.

At this point, students should be comfortable multiplying side lengths of rectangles to find the area. Discussion should focus on how students decompose the figures and how they find the missing side lengths.

*MLR8 Discussion Supports:*Before students explain their strategies to each other, remind them to restate what they hear using precise mathematical language and their own words. Display the sentence frame: “I heard you say . . . .” Original speakers can agree or clarify for their partner.

*Advances: Listening, Speaking*

*Engagement: Provide Access by Recruiting Interest:*Use timers, alerts, or previews to help learners anticipate and prepare to transition between activities.

*Supports accessibility for: Attention, Organization*

### Launch

- Groups of 2

### Activity

- “Now that we’ve discussed how we would find the missing side length of this figure, find the area of this figure.”
- 3–5 minutes: independent work time
- Monitor for a variety of strategies to group students for discussion.
- “Find someone who used the same strategy that you used to find the area of the figure. Discuss how your strategy makes sense.”
- 2 minutes: partner discussion
- “Now, find someone who used a different strategy than you. Explain your strategies to each other. Be sure you could explain your partner’s strategy to the class.”
- 4 minutes: partner discussion
- “Work with your current partner to find the area of the second figure.”
- 5 minutes: partner work time
- Circulate and consider asking: “What information do you know that might help you find what you are missing?”
- Monitor for a variety of ways that students use to find the area of the second figure, including different decompositions, different strategies for finding missing sides lengths, and different expressions they write.

### Student Facing

Find the area of each figure. Explain or show your reasoning.

### Student Response

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### Advancing Student Thinking

If students determine incorrect lengths for the missing sides, consider asking:

- “Tell me about how you found the missing side length.”
- “What strategies from the previous activity could you use to find the missing side length?”

### Activity Synthesis

- Select students to share a variety of strategies for finding the area of the second figure.
- Consider asking:
- “What missing side had to be found to use this strategy? How did you find the missing side length?”
- “After you knew the missing side lengths, how did you find the area?”
- “What is the expression we would write to match how you decomposed the figure?”
- “Is there another way that we could break this figure into non-overlapping rectangles?” (Students may also see the figure as 3 rectangles: \((2 \times 3) + (6 \times 2) + (2 \times 2)\).)

## Lesson Synthesis

### Lesson Synthesis

Display the figure from the warm-up.

“Today we learned that we can find missing side lengths in figures. What is it about the structure of the figures we saw in this lesson that helps you find missing side lengths?” (They are made up of rectangles, so if we break them into rectangles, we can think about how the lengths of opposite sides need to match.)

“How do you like to find the area of these figures after you know the side lengths?” (I break the shape into rectangles I see and find their area. Then I add the area of those rectangles together.)

## Cool-down: Mystery Side Area (5 minutes)

### Cool-Down

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## Student Section Summary

### Student Facing

In this section, we found the area of figures that could be decomposed into rectangles. We added the area of each rectangle to find the area of the entire figure.

We also found missing side lengths by using what we know about opposite sides of rectangles.