Lesson 10

The purpose of this warm-up is to introduce students to cube roots during the discussion. This activity provides an opportunity to use cube root language and notation during the discussion. Students will explore the possibility of negative cube roots in the next lesson.

At first, students should be able to order the values of \(a\), \(b\), \(c\), and \(f\) without a calculator. However, \(d\) and \(e\) will be easier with a calculator. Encourage students to use estimated values for \(d\) and \(e\) to order the values before using a calculator. As students work, identify students who use different strategies for ordering.

After introducing students to cube root language in the activity synthesis, display this text, image, and table for all to see:

Fill in the missing values using the information provided:

sides	volume	volume equation
	\(27\,\text{in}^3\)
\(\sqrt[3]{5}\)
		\((\sqrt[3]{16})^3=16\)

Tell students that the table shows the values for three different cubes where all values are exact. Give students 1–2 minutes of work time with their partner to identify as many missing values as they can. Select groups to share their reasoning about which values go into the blanks on the table.

Previously, students learned that knowing the area of a square is sufficient information for stating the exact length of the side of the square using square roots. The purpose of this discussion is for students to make that same connection with the volume of cubes and cube roots. The cube root of the volume of a cube represents the exact value of the edge length of a cube in ways that measuring or approximating by cubing values do not (except in special cases such as perfect cubes).

If time allows, discuss the following questions:

• “What integers is \(\sqrt[3]{5}\) between?” (\(\sqrt[3]{5}\) is between 1 and 2 because \(1^3 = 1\) and \(2^3 = 8\).)
• “What integers is \(\sqrt[3]{16}\) between?” (\(\sqrt[3]{16}\) is between 2 and 3 because \( 2^3 = 8\) and \(3^3 = 27\).)
• “Name another volume for a cube with edge lengths between 2 and 3.” (A cube with volume 26 has edge lengths between 2 and 3 since since \(3^3=27\), meaning \(\sqrt[3]{26}\) has a value slightly less than 3.)

Students in groups of 2. Give students 2–3 minutes to order the options to the best of their ability without a calculator, and to share their reasoning with a partner. Pause to discuss which are easy to order (likely \(f\), \(b\), \(c\), and \(a\)) and which ones students are not sure about (likely \(d\) and \(e\), which are between \(b\) and \(a\)). Then give students 1 minute with a calculator to finish ordering the options. Follow with a whole-class discussion.

Ask a student to share their order of \(a\), \(b\), \(c\), \(d\), \(e\), and \(f\) from least to greatest. Record and display their responses for all to see. Ask the class if they agree or disagree. If the class agrees, select previously identified students to share their strategies for ordering the values. If the class is in disagreement, ask students to share their reasoning until an agreement is reached.

As students share, ask students which were the easiest to find and which were the hardest to find. Introduce students to cube root language and notation. Remind students that they previously learned that the equation \(c^2=10\) has solution \(c=\sqrt{10}\). Similarly, we can say that the equation \(d^3=9\) has solution \(d=\sqrt[3]{9}\). Ask students to write the solution to
\(f^3=7\) (\(f=\sqrt[3]{7}\)).

Finally, tell students that while square roots are a way to write the exact value of the side length of a square with a known area, cube roots are a way to write the exact value of the edge length of a cube with a known volume, which students will do in a following activity.

The purpose of this activity is for students to use rational approximations of irrational numbers and to match irrational numbers to equations they are solutions to. To do this, students sort cards into sets of three consisting of:

• A square or cube root value
• An equation of the form \(x^2=p\) or \(x^3=p\) that the value is a solution to
• A number line showing the location of the value

Identify groups using clear explanations for how they chose to arrange their sets of 3, particularly when matching the cards with a value plotted on a number line.

Arrange students in groups of 2–4. Students should not use a calculator for this activity. Tell students that they will be given cards to sort into sets of three. Distribute 27 pre-cut slips from the blackline master to each group. Group work time followed by a whole-class discussion.

Some students may mix up cube and square roots. Encourage these students to pay close attention to the notation. They may want to fill in the “2” for the square roots similar to how the “3” is used for cube roots.

Select 3–4 previously identified groups to share one of their sets of three cards.

Also discuss:

• “What was a match that was hard to make?” (Two of the number lines have values between 4 and 5, and it took some extra reasoning to figure out that the one with the value closer to 4 was \(\sqrt{18}\) while the one with the point closer to 5 was \(\sqrt[3]{100}\).
• “If another class was going to sort these cards, what is something you would recommend they have or do that you found helpful?” (I would recommend they have a list of perfect squares and perfect cubes to help think about where the different roots are plotted on the number line.)

The purpose of this activity is for students to think about cube roots in relation to the two whole number values they are closest to. Students are encouraged to use the fact that \(\sqrt[3]{a}\) is a solution to the equation \(x^3=a\). Students can draw a number line if that helps them reason about the magnitude of the given cube roots, but this is not required. However students reason, they need to explain their thinking (MP3).

Monitor students multiplying non-integers by hand to try and approximate. While this isn’t what the problem is asking for, their work could be used to think about which integer the square root is closest to and should be brought up during the whole-class discussion.

Do not give students access to calculators. Students in groups of 2. Two minutes of quiet work time, followed by partner then whole-class discussion.

Discuss:

• “What strategy did you use to figure out the two whole numbers?”
  (I made a list of perfect cubes and then found which two the number was between.)
• “How can we write one of these answers using inequality symbols?” (For example, \(2<\sqrt[3]{23}<3\).)

The purpose of this activity is for students to use rational approximations of irrational numbers to place both rational and irrational numbers on a number line, and to reinforce the definition of a cube root as a solution to the equation of the form \(x^3=a\). This is also the first time that students have thought about negative cube roots. 

No access to calculators. Students in groups of 2. Two minutes of quiet work time, followed by partner then whole-class discussion.

Display the number line from the activity for all to see. Select groups to share how they chose to place values onto the number line. Place the values on the displayed number line as groups share, and after each placement poll the class to ask if students used the same reasoning or different reasoning. If any students used different reasoning, invite them to share with the class.

Conclude the discussion by asking students to share how they placed -\(\sqrt[3]{2}\) on the number line.