Lesson 16

The purpose of this activity is to introduce students to the central problem of the lesson: how to calculate which cylinder takes the least material to build for a specific volume. Before students begin working with surface areas of cylinders, they are explicitly asked to make an estimate. Making a reasonable estimate and comparing a computed value to one’s estimate is often an important aspect of making sense of problems (MP1).

Before students do calculations or any other work, display the image of the cylinders and poll the class about which option they think requires the least material to build. Display the results of the poll for all to see.

The purpose of this discussion is to make sure students remember how to calculate the volume and surface area of a cylinder.

Select students who chose A, B, C, and D to share their reasoning for why that cylinder requires the least material to build. If not brought up during the discussion, discuss how to calculate the volume of a cylinder \(V=\pi r^2h\) and the surface area of a cylinder \(S=2\pi r^2 +2\pi rh\) and write these formulas for all to see where they can remain displayed throughout the lesson. Consider using a rolled piece of paper to demonstrate the rectangular part of the surface area. If students are unsatisfied with not knowing which of the 4 has the smallest surface area, assure them that we will revisit the problem at the end of the lesson.

In this activity, students calculate the height needed for a cylinder with a specific volume and radius. The number of calculations needed to fill in the table is purposeful in order to encourage students to rearrange the formula for the volume of a cylinder to best suit their needs (MP8). The table and subsequent graph also help students identify that as one value increases (radius), the other decreases (height). The last question is meant to elicit student language about the general shape of the graph as well as allude to a topic that will be addressed directly in future lessons: horizontal asymptotes.

Monitor for students who rearrange the formula for the volume of a cylinder to share during the discussion.

Arrange students in groups of 2. Give groups 4–5 minutes of work time and then pause the class, selecting 1–2 previously identified students to share how they did the calculations for height by rearranging the formula for the volume of a cylinder as \(h=\frac{\text{volume}}{\pi r^2}\) and record this for all to see next to the equations recorded from the previous activity.

Provide access to devices that can run Desmos or other graphing technology.

Some students may mistakenly interpret \(\pi r^2\) in the volume formula as \((\pi r)^2\). Remind these students that, without parentheses to include \(\pi\), the squaring only applies to \(r\).

Invite students to share things they noticed about the graphs they created. Here are some questions for discussion to prime students for the idea of asymptotes, which will be introduced in the next lesson.

• “Are there any cylinders for which the point \((r,h)\) could be on an axis?” (No, if one of the dimensions is 0, then the cylinder has no volume and does not exist.)
• “Where would the point be if the value of \(r\) was very, very small?” (If \(r\) is very, very small, then \(h\) would be very, very big, so the point would be close to the \(h\) axis and very far from the \(r\) axis in the positive direction.)

The purpose of this activity is for students to combine formulas together to create an equation for the function relating the radius and surface area of cylinders with a specific volume, and graph it to learn more about it. Students also learn that the relationship they are investigating is called a rational function along with some features of equations of rational functions.

Arrange students in groups of 2. Tell half of the groups to calculate the surface area of a cylinder with radius 2 cm and the other half to calculate the surface area of a cylinder with radius 3 cm and to put their calculations into the table. Give groups 3–5 minutes of work time, and then select groups to share their calculations. It turns out that the surface area when \(r=3\) is smaller than when \(r=2\). Tell students to discuss a strategy with their partner to figure out the radius that leads to the smallest surface area for the cylinder and then use the table to record their work. Provide access to devices that can run Desmos or other graphing technology.

If students are unsure of how to write \(S\) as a function of \(r\), it may help them to start with the equation for surface area. This equation involves \(h\), which will need to be eliminated in order to find an equivalent expression that only involves \(S\) and \(r\). Remind students that in the previous activity, they expressed \(h\) in terms of \(r\), and that this can help them write \(S\) in terms of \(r\).

The goal of this discussion is to name the relationship students have investigated in this lesson and identify the dimensions of the cylinder with volume 452 cm³ that has the smallest surface area, and so takes the least amount of materials to make.

If time allows, pair groups to share their graphs and observations before the whole-class discussion. Otherwise, begin by inviting 2–3 groups to share their graphs and things they noticed about the graphs, recording responses for all to see. Here are some questions for discussion.

• “What values of radius and height result in the smallest surface area?” (A radius of about 4.2 cm with a height of 8.2 cm minimizes the surface area.)
• “What is the minimum surface area and why would we care about finding this value?” (The minimum surface area is about 326.1 cm². Less surface area means less material used to make the cylinder, and that could make the cylinder cheaper to build for, say, a can manufacturer.)
• “How did you figure out the equation for the relationship between \(S\) and \(r\)?” (We already knew that \(h=\frac{452}{\pi r^2}\) for these cylinders, so replacing the \(h\) values in the equation for surface area gives a new equation with only \(r\) and \(S\).)

Tell students that the relationship between the height and volume and between the surface area and radius of the cylinder are examples of rational functions. Rational functions include polynomials, but allow fractions with polynomials in the numerator and denominator (so long as the denominator isn’t 0). If time allows, show students how to rewrite \(S=2\pi r^2+\frac{904}{r}\) as \(S=\frac{2\pi r^3 + 904}{r}\), which looks more like a polynomial divided by a polynomial.