Lesson 9
Applying Area of Circles
Lesson Narrative
In previous lessons, students estimated the area of circles on a grid and explored the relationship between the circumference and the area of a circle to see that \(A = \pi r^2\). In this lesson, students apply this formula to solve problems involving the area of circles as well as shapes made up of parts of circles (MP 1 and 2) and other shapes such as rectangles. These calculations require composition and decomposition recalling work from grade 6.
Also, in this lesson students are introduced to the idea of expressing exact answers in terms of \(\pi\).
Learning Goals
Teacher Facing
 Calculate the area of a shape that includes circular or semicircular parts, and explain (orally and in writing) the solution method.
 Comprehend and generate expressions in terms of $\pi$ to express exact measurements related to a circle.
Student Facing
Let’s find the areas of shapes made up of circles.
Required Materials
Required Preparation
It is recommended that fourfunction calculators be made available to take the focus off computation.
Learning Targets
Student Facing
 I can calculate the area of more complicated shapes that include fractions of circles.
 I can write exact answers in terms of $\pi$.
CCSS Standards
Addressing
Glossary Entries

area of a circle
If the radius of a circle is \(r\) units, then the area of the circle is \(\pi r^2\) square units.
For example, a circle has radius 3 inches. Its area is \(\pi 3^2\) square inches, or \(9\pi\) square inches, which is approximately 28.3 square inches.

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 \(s^2\).