Circles

Learn to find the area and circumference of circles.

A circle is the set of points in a plane that are a fixed distance from a given point, called the center.

A radius connects the center to any point on the circle.

A diameter connects two points on the circle and passes through the center.

Radius

CenterDiameter

Circumference

The diameter d is twice the radius r.

d = 2r

The circumference of a circle is the distance around the circle.

A. Circle with a radius of 4 m

C = 2pr= 2p(4)

= 8 p m

B. Circle with a diameter of 3.3 ft

C = pd= p(3.3)

= 3.3 p ft

Find the circumference of each circle in terms of

.4m

3.3ft

A = pr2 = p(42)= 16 p in2

A. Circle with a radius of 4 in.

Find the area of each circle in terms of .p

B. Circle with a diameter of 3.3 m

A = pr2 = p(1.652)

= 2.7225p m2

d2 = 1.65

4in

3.3m

Tweedle Dum & Tweedle Dee will help you remember your

circle formulas!

Tweedle Dum and Tweedle Dee

Around the circle is pi times d.

And if the area is declared

Then its pi r squared.

A = pr2

= p(32)

= 9 p units2

C = pd

= p(6)

= 6 p units

Graph the circle with center (–2, 1) that passes through (1, 1). Find the area and circumference, in terms of .p

C = pd = p(56)

176 ft (56) 22 7

22 7

A Ferris wheel has a diameter of 56 feet and makes 15 revolutions per ride. How far would someone travel during a ride? Use for p.

Find the circumference.

56 1

22 7

The distance is the circumference of the wheel times the number of revolutions, or about 176 15 = 2640 ft.

Lesson Quiz

Find the circumference of each circle in terms of .p

1. radius 5.6 m

2. diameter 113 m

11.2 p m

113 p mm

Find the area of each circle in terms of p.

3. radius 3 in.

4. diameter 1 ft

9 p in2

0.25 p ft2