This document provides information about calculating the area of circles. It defines circumference and diameter, and presents the formulas for calculating the circumference and area of a circle using pi and the radius. It includes examples of using the area formula to calculate the area when given the radius or diameter. The final challenge is to calculate the area of a donut by finding the area of the larger circle and subtracting the area of the smaller circle.

1. 11-6
Area of Circles
Standard P8 Use formulas in problem-
solving situations
Page 493-495

2. Circumference of a Circle
The distance around a circle is called its
circumference.
The distance across a circle through its center
is called its diameter.
We use the Greek letter (pronounced Pi) to
represent the ratio of the circumference of a circle
to the diameter.
The formula for circumference of a circle is:
(We typically use 3.14 for Pi)
The diameter of a circle is twice as long as the radius.
d
C 


3. Area of a Circle
The area of a circle is the number of square
units inside that circle.
If each square in the circle to the left has an
area of 1cm2, you could count the total
number of squares to get the area of this
circle.
Thus, if there were a total of 28.26 squares,
the area of this circle would be 28.26 cm2
However, it is easier to use one of the following
formulas: ,where A=area, and
r=radius.
2
r
A 


4. Example 1
The radius of a circle is 3 inches. What is
the area? Round to the nearest
hundredth, if necessary.
= 3.14 · (3 in)2
= 3.14 · (9 in2)
= 28.26 in2
2
r
A 


5. Example 2
The diameter of a circle is 8 centimeters. What
is the area? Round to the nearest
hundredth if necessary.
Remember: you need the
radius measurement.
2
r
A 

2
)
4
( cm
A 

)
16
( 2
cm
A 

2
24
.
50 cm
A 

6. Example 3
The area of a circle is 78.5 square meters.
What is the radius?
2
r
A 

2
5
.
78 r





2
5
.
78 r

2
25 r

r

25
r

5

7. Challenge
Find the area of the donut.
Hint: find the area of both circles & subtract.
4 in
1 in
2
r
A 
