Lesson 18

The purpose of this warm-up is for students to review how to compute the area of a circle. This idea should have been carefully developed in grade 7. This warm-up gives students an opportunity to revisit this idea in preparation for finding the volume of a cylinder later in the lesson.

Students begin the activity with a whole-class discussion in which they identify important features of a circle including its radius and diameter. They use this information and the formula for the area of the circle to choose expressions from a list that are equivalent to the area of the circle. In the final question, students are given the area of the circle and find the corresponding radius. 

As students are working, monitor for students who can explain why \(16\pi\), \(\pi 4^2\), and “approximately 50” square units represent the area of the circle.

Display the diagram from the task statement for all to see and ask students:

• “Name a segment that is a radius of circle A.” (AC, AD, and AB or those with the letters reversed are all radii.) Review the meaning of the radius of a circle.
• “What do we call a segment like BC, with endpoints on the circle that contains the center of the circle?” (A diameter.) Review the meaning of a diameter of a circle.
• “What is the length of segment \(AB\)?” (4 units.) Review the fact that all radii of a circle have the same length.

Give students 3 minutes of quiet work time and follow with a whole-class discussion.

If students struggle to recall how to find the area of a circle, encourage them to look up a formula using any available resources.

The purpose of this discussion is to make sure students remember that the area of a circle can be found by squaring its radius and multiplying by \(\pi\).

Select previously identified students to share answers to the first question and explain why each of the solutions represents the area of the circle. If not brought up during the discussion, tell students that sometimes it is better to express an area measurement in terms of \(\pi\). Other times it may be better to use an approximation of \(\pi\), like 3.14, to represent the area measurement in decimal form. In this unit, we will often express our answers in terms of \(\pi\).

Display in a prominent place for all to see for the next several lessons: Let \(A\) be the area of a circle of radius \(r\), then \(A=\pi r^2\).

The purpose of this activity is for students to connect their previous knowledge of the volume of rectangular prisms to their understanding of the volume of cylinders. From previous work, students should know that the volume of rectangular prisms is found by multiplying the area of the base by the height. Here we expand upon that to compute the volume of a cylinder.

Students start by calculating the volume of a rectangular prism. Then they extrapolate from that to calculate the volume of a cylinder given the area of its base and its height. If some students don’t know they should multiply the area of the base by its height, then they are prompted to connect prisms and cylinders to make a reasonable guess.

We want students to conjecture that the volume of a cylinder is the area of its base multiplied by its height. 

Arrange students in groups of 2. Remind students that a rectangular prism has a base that is a rectangle, and a cylinder has a base that is a circle. It may have been some time since students have thought about the meaning of a result of a volume computation. Consider showing students a rectangular prism built from 48 snap cubes with the same dimensions as Figure A. It may help them to see that one layer is made of 16 cubes. Give students 3–5 minutes of quiet work time followed by a partner discussion. During their discussion, partners compare the volumes they found for the cylinders. If they guessed the volumes, partners explain their reasoning to one another. Follow with a whole-class discussion.

Highlight the important features of cylinders and their definitions: the radius of the cylinder is the radius of the circle that forms its base; the height of a cylinder is the length between its circular top and bottom; a cylinder of height 1 can be thought of as a “layer” in a cylinder with height \(h\). To highlight the connection between finding the area of a rectangular prism and finding the area of a cylinder, ask:

• “How are prisms and cylinders different?” (A prism has a base that is a polygon, and a cylinder has a base that is a circle.)
• “How are prisms and cylinders the same?” (The volume of cylinders and prisms is found by multiplying the area of the base by the height. \(V=Bh\))
• “How do you find the area of the base, \(B\), of a cylinder?” (\(B=\pi r^2\)).

In this activity, students find the missing dimensions of cylinders when given the volume and the other dimension. A volume equation representing the cylinder is given for each problem.

Identify students who use these strategies: guess and check, divide each side of the equation by the same value to solve for missing variable, or use the structure of the volume equation to reason about the missing variable

Arrange students in groups of 2. Give students 2–3 minutes of quiet work time followed by time to share their explanation for the first problem with their partners.

Select previously identified students to explain the strategies they used to find the missing dimension in each problem. If not brought up in students’ explanations. Discuss the following strategies and explanations:

• Guess and check: plug in numbers for \(h\), a value that make the statements true. Since the solutions for these problems are small whole numbers, this strategy works well. In other situations, this strategy may be less efficient.
• Divide each side of the equation by the same value to solve for the missing variable: for example, divide each side of \(36\pi=r^2 \pi 4\) by the common factor, \(4\pi\). It’s important to remember \(\pi\) is a number that can be multiplied and divided like any other factor.
• Use the structure of the equation to reason about the missing variable: for example, \(50\pi\) is double \(25\pi\), so the missing value must be 2.

The purpose of this activity is for students to use the structure of the volume formula for cylinders to find missing dimensions of a cylinder given other dimensions. Students are given the image of a generic cylinder with marked dimensions for the radius, diameter, and height to help their reasoning about the different rows in the table..

While completing the table, students work with approximations and exact values of \(\pi\) as well as statements that require reasoning about squared values. The final row of the table asks students to find missing dimensions given an expression representing volume that uses letters to represent the height and the radius. This requires students to manipulate expressions consisting only of variables representing dimensions.

Encourage students to make use of work done in some rows to help find missing information in other rows. Identify students who use this strategy and ask them to share during the discussion.

Give students 6–8 minutes of work time followed by a whole-class discussion.

If short on time, consider assigning students only some of the rows to complete.

Students might try to quickly fill in the missing dimensions without the proper calculations. Encourage students to use the volume of a cylinder equation and the given dimensions to figure out the unknown dimensions.

Select previously identified students to share their strategies. Ask students:

• “What patterns did you see as you filled out the table?” (Sample reasoning: Rows that had the same base area were easier to compare because their volume was the base area times height.)
• “Look at rows 1 and 3 in the table. How did having one row filled out help you fill out the other more efficiently?” (If the base areas were the same, then the radius and diameter must be the same also.)
• “How did you reason about the last row?”