# Lesson 18

Surface Area of a Cube

### Lesson Narrative

In this lesson, students practice using exponents of 2 and 3 to express products and to write square and cubic units. Along the way, they look for and make use of structure in numerical expressions (MP7). They also look for and express regularity in repeated reasoning (MP8) to write the formula for the surface area of a cube. Students will continue this work later in the course, in the unit on expressions and equations.

Note: Students will need to bring in a personal collection of 10–50 small objects ahead of time for the first lesson of the next unit. Examples include rocks, seashells, trading cards, or coins.

### Learning Goals

Teacher Facing

• Generalize a process for finding the surface area of a cube, and justify (orally) why this can be abstracted as $6 \boldcdot s^2$.
• Interpret (orally) expressions that include repeated addition, multiplication, repeated multiplication, or exponents.
• Write expressions, with or without exponents, to represent the surface area of a given cube.

### Student Facing

Let’s write a formula to find the surface area of a cube.

### Student Facing

• I can write and explain the formula for the surface area of a cube.
• When I know the edge length of a cube, I can find its surface area and express it using appropriate units.

### Glossary Entries

• cubed

We use the word cubed to mean “to the third power.” This is because a cube with side length $$s$$ has a volume of $$s \boldcdot s \boldcdot s$$, or $$s^3$$.

• exponent

In expressions like $$5^3$$ and $$8^2$$, the 3 and the 2 are called exponents. They tell you how many factors to multiply. For example, $$5^3$$ = $$5 \boldcdot 5 \boldcdot 5$$, and $$8^2 = 8 \boldcdot 8$$.

• squared

We use the word squared to mean “to the second power.” This is because a square with side length $$s$$ has an area of $$s \boldcdot s$$, or $$s^2$$.

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