This document defines key terms and formulas related to circles, including circumference and area. It defines a circle as all points equidistant from a given center point. The radius is the distance from the center to the edge, and the diameter runs through the center. Circumference is defined as the distance around the circle and is calculated using either C=2πr or C=πd. Area is calculated as A=πr^2. Several examples are provided to demonstrate calculating circumference and area using these formulas.

1. Circles: Circumference & Area
Circle: is the set of all points in
a plane that are equidistant from
a given point, called the center of
the circle.
Radius: the distance from the
center to a point on the circle.
Diameter: distance across the
circle, through the center.
r
Circumference: the
distance around the circle
(just like perimeter)
Formulas: C = 2πr or C = πd
π: Pi – the ratio of the circles
circumference to its diameter ≈ 3.14…

2. Circumference Formulas:
C = 2πr or C = πd
π is about 3.14…use symbol in calculator
EX Find the circumference of the following circles. Round
your answer to the nearest tenth.
2 C = 2πr
C = 2•π•2
C = 4π ≈ 12.6
10
C = πd
C = π•10
C = 10π ≈ 31.4
EX The circumference of a circle is 10, What is its diameter
in terms of π ?
π
10
diameter =
EX The radius of a circle is 8, What is the circumference in
terms of π ? C = 16 π

3. Area of a Circle Area = πr2
EX Find the area of the following circle. Round to the nearest
tenth.
7
A= π72
A= 49π
A = 153.9 units2
20 A = 314.2
units squared
EX The area of a circle is given as 95 square units. Find the
diameter of the circle, to the nearest whole number.
Diameter = 11 units
EX The area of a circle is given as 25π. Find the
circumference in terms of π. A = πr2
25π=πr2
25 = r2
5 = r
C = 2πr
C = 2•π•r
C = 10π

4. EX
More Examples:
The circumference of a circle is 18π. Find the
area of the circle, leave answer in terms of π.
C = 2 π r
18π = 2πr
9 = r
A = πr2
A = π92
A = 81π