Lesson 11

This activity introduces the concept of perfect squares. It also includes opportunities to practice using units of measurement, which offers insights on students’ knowledge from preceding lessons. 

Provide access to square tiles, if available.  Some students may benefit from using physical tiles to reason about perfect squares.

As students work, notice whether they use appropriate units for the second and third questions. 

Tell students, “Some numbers are called perfect squares. For example, 9 is a perfect square. Nine copies of a small square can be arranged into a large square.” Display a square like this for all to see:

Explain that 10, however, is not a perfect square. Display images such as these below, emphasizing that 10 small squares can not be arranged into a large square (the way 9 small squares can).

Tell students that in this warm-up they will find more numbers that are perfect squares. Give students 2 minutes of quiet think time to complete the activity.

If students do not recall what the abbreviations km, cm, and sq stand for, provide that information.

Students may divide 64 by 2 for the third question. If students are having trouble with this, ask them to check by working backwards, i.e. by multiplying the side lengths to see if the product yields the given area measure.

Invite students to share the examples and non-examples they found for perfect squares. Solicit some ideas on how they decided if a number is or is not a perfect square.

If a student asks about 0 being a perfect square, wait until the end of the lesson, when the exponent notation is introduced. 0 is a perfect square because \(0^2 = 0\).

Briefly discuss students’ responses to the last two questions, the last one in particular. If not already uncovered in discussion, highlight that because the area of a square is found by multiplying side lengths to each other, finding the side lengths of a square with a known area means figuring out if that area measure is a product of two of the same number.

This activity gives students a concrete way to review the work on volume from grade 5. It prompts students to recall that the volume of a rectangular prism can be calculated in two different ways: by counting unit cubes that can be packed into the prism, and by multiplying the edge lengths of the prism. Students also become familiar with two perfect cubes, 27 and 64, before the next activity introduces this term.

As students work, monitor the different routes they take to find the volume of the built cube. They may count all of the snap cubes individually, count the number of snap cubes per layer and then multiply that by the number of layers, or simply multiply edge lengths. Select a student who uses each method to share later.

Students may neglect to write units for length or area and may need a reminder to do so.

When determining area, students may multiply a side by two instead of squaring it. When determining volume, they may multiply a side by three instead of cubing it. If this happens, ask them to count individual squares so that they can see that there is an error in their reasoning.

Focus the whole-class discussion on the ways students calculated the volumes of the two cubes they built. Select previously identified students to share their approaches starting from the less efficient (counting individual cubes) to the most efficient (multiplying side lengths).

Highlight how the side length of a cube determines its volume, and specifically that the number 27 is \(3\boldcdot 3\boldcdot 3\). If any group built a cube with 64 snap cubes, point out that the number 64 is \(4\boldcdot 4\boldcdot 4\). These observations prepare students to think about perfect cubes in the next activity and about a general expression for the volume of a cube later in the lesson.

Earlier, students looked at examples and non-examples of perfect squares. In this activity, they think about examples and non-examples of perfect cubes and find the volumes of cubes given their edge lengths. Students see that the edge length of a cube determines its volume, notice the numerical expressions that can be written when calculating volumes, and write a general expression for finding the volume of a cube (MP8).

Some students may feel uncomfortable writing the answer to the last question symbolically because it involves a variable and may prefer writing a verbal explanation. This is fine; the exponential notation that follows will help greatly.

Tell students, “Some numbers are called perfect cubes. For example, 27 is a perfect cube.” Display a cube like this for all to see:

Arrange students in groups of 2. Give students a few minutes of quiet think time, and another minute to discuss their responses with their partner.

Watch for students using square units instead of cubic units. Remind them that volume is a measure of the space inside the cube and is measured in cubic units.

Students may multiply by 3 when finding the volume of a cube instead of multiplying three edge lengths (which happen to be the same number). Likewise, they may think a perfect cube is a number times 3. Suggest that they sketch or build a cube with that edge length and count the number of unit cubes. Or ask them to think about how to find the volume of a prism when the edge lengths are different (e.g., a prism that is 1 unit by 2 units by 3 units).

After partner discussions, invite students to share how they thought about the first question and decided if a number is or is not a perfect cube. Highlight the idea that multiplying three edge lengths allows us to determine volume efficiently, and that determining if a number is a perfect cube involves thinking about whether it is a product of three of the same number.

If a student asks about 0 being a perfect cube, wait until the end of the lesson, when exponent notation is introduced. 0 is a perfect cube because \(0^3 = 0\).

Make sure students see the answers to the last three questions written as expressions: \(\displaystyle 4 \boldcdot 4 \boldcdot 4\) \(\displaystyle 10 \boldcdot 10 \boldcdot 10\) \(\displaystyle s \boldcdot s \boldcdot s\)

This activity introduces students to the exponents of 2 and 3 and the language we use to talk about them. Students use and interpret this notation in the context of geometric squares and their areas, and geometric cubes and their volumes. Students are likely to have seen exponent notation for \(10^3\) in their work on place values in grade 5. That experience would be helpful but is not necessary.

Note that the term “exponent” is deliberately not defined more generally at this time. Students will work with exponents in more depth in a later unit.

As students work, observe how they approach the last two questions. Identify a couple of students who approach the fourth question differently so they can share later. Also notice whether students include appropriate units, written using exponents, in their answers.

Ask students if they have seen an expression such as \(10^3\) before. Tell students that in this expression, the 3 is called an exponent. Explain the use of exponents of 2 and 3:

• “When we multiply two of the same number together, such as \(5 \boldcdot 5\), we say we are squaring the number. We can write the expression as: \(\displaystyle 5^2\) Because \(5\boldcdot 5\) is 25, we can write \(5^2 = 25\), and we say, ‘5 squared is 25.’ We can also say that 25 is a perfect square. The raised 2 in \(5^2\) is called an exponent.”
• “When we multiply three of the same number together like \(4 \boldcdot 4 \boldcdot 4\), we say we are cubing the number. We can write it like this: \(\displaystyle 4^3\) Because \(4 \boldcdot 4 \boldcdot 4\) is 64, we can write \(4^3 = 64\), and we say, ‘4 cubed is 64.’ We also say that 64 is a perfect cube. The raised 3 in \(4^3\) is called an exponent.”

Explain that we can also use exponents as a shorthand for the units used for area and volume:

• A square with side length 5 inches has area of 25 square inches, which we can write as 25 in².
• A cube with edge length 4 centimeters has a volume of 64 cubic centimeters, which we can write as 64 cm³.

Ask students to read a few areas and volumes in different units (e.g. 100 ft² is read “100 square feet” and 125 yd³ is read “125 cubic yards”).

Keep students in groups of 2. Give students 3–4 minutes of quiet time to complete the activity and a minute to discuss their response with their partner. Ask partners to note any disagreements so they can be discussed.

Upon seeing \(6^3\) in the fifth question, some students may neglect to interpret the question, automatically calculate \(6\boldcdot 6\boldcdot 6\), and conclude that the edge length is 216 cm. Ask them to check their answer by finding the volume of a cube with edge length 216 cm. 

Ask partners to share disagreements in their responses, if any. Then, focus the whole-class discussion on the last two questions. Select a couple of previously identified students to share their interpretations of the fourth question.

Highlight that a cube with a volume of \(6^3\) cubic units has an edge length of 6 units, because we know there are \(6 \boldcdot 6 \boldcdot 6\) unit cubes in a cube with that edge length.

In other words, we can express the volume of a cube using a number (216), a product of three numbers (\(6 \boldcdot 6 \boldcdot 6\)), or an expression with exponent (\(6^3\)). This idea can be extended to all cubes. The volume of a cube with edge length \(s\) is: \(\displaystyle s \boldcdot s \boldcdot s\) \(\displaystyle s^3\) Students will have more opportunities to generalize the expressions for the volume of a cube in the next lesson.