How do you find the area of a circle?

For Example

A circle has a diameter of 10 meters. What is the area of the circle?

In this lesson you will learn to find the area of a circle by exploring the relationship

between a circle and a rectangle.

Let’s ReviewLet’s Review

Area of Rectangle = l x w

A = 5cm x 9cm

5 cm

9 cm

A = 45 cm2

Let’s ReviewLet’s Review

Multiplying a number or variable by itself is known as squaring it.

9 x 9 = 92 = 81

r x r = r2

Let’s ReviewA Common Mistake

Remember the Order of Operations. Exponents come before multiplication.

A = (3.14)(4)2

A = (12.56) 2

Wrong

A = 157.7536

Correct

A = (3.14)(16)A = 50.24

Let’s ReviewCore Lesson

A pizza has a radius of 9 inches. What is the area of the pizza?

r = 9 inches

Let’s ReviewCore Lesson

Area of a Circle =πr2

πr

r

Let’s ReviewCore Lesson

A = π(9in.)2

A = 3.14(81in.2)

A = 254.34in.2

Let’s ReviewCore Lesson

Find the area of a circle with a diameter of 10 meters. Do not approximate pi.

A = π(5m)2

A = 25πm2

In this lesson you have learned to find the area of a circle by

exploring the relationship between circles and

rectangles.

Let’s ReviewGuided Practice

What is the area of a circle that has a radius of 6 cm? Use 3.14 for pi.

Let’s ReviewExtension Activities

Susan drew a circle with a radius of 4 inches and Ellen drew a circle with a radius of 8 inches. Ellen said “since the radius of my circle with twice the radius of your circle, the area of my circle is twice the area of your circle.” Is Ellen’s statement correct? If not, explain what Ellen could say instead.

Let’s ReviewExtension Activities

A circle is inside of a square, as shown below. The edges of the square each touch a point on the circle. If the square has an area of 16 square meters, what is the area of the circle?

Let’s ReviewExtension Activities

A circular pizza can feed 4 people if it has an area of at least 200 square inches. A pizza

from Joe’s Pizza has a radius of 9 inches. Is it enough to feed a family of 4?