Circle is a closed plane
figure in which all the
points are the same
distance from the center.
A
Center
F
Circle F
A
Chord is a line segment
that connects two points on
a circle.
B
A
C
Y
D
Chords: AB
AC
AD
The Diameter of a circle is
a special chord that passes
through the center of a
circle. B
A
C
E
D
Diameter: AC
The Radius of a circle is a
line segment from the
center of the circle to any
other point on the circle.
B
Y
C
A
D
Radii: YA
(plural YC
form of
radius) YD
The length of
the radius is one
half the length
of the diameter.
d=2xr
or
d (diameter)
r (radius)
r=d÷2
If r = 2 cm, then d = 4 cm.
If d = 5 in, then r = 2.5 in.
You can use a
compass to
construct a circle.
compass
Step 1: Open the compass
to the length of the radius.
Step 2: Put the tip of the
compass at a point to be
the center.
Step 3: Rotate the pencil
around the center.
P
The Circumference is the
distance around a circle.
C (circumference)
The Circumference of a circle
is greater than its diameter. It is
always the same number of times as
great. This number is called pi and
is represented by the symbol π.
( π  3.14)
You can find the circumference by
multiplying π times the diameter.
(C=πxd)
d = 8 cm
E
8 cm
F
G
C=πxd
= 3.14 x 8
= 25.12 cm
The circumference is 25.12 cm.
r = 3 in
3 in
Remember:
d=2xr
S
T
C=πxd
C=2xπxr
= 2 x 3.14 x 3
= 18.84 in
The circumference is 18.84 in.
C = 62 m
Find the diameter (d) to the nearest
hundredth.
C=πxd
62 = 3.14 x d
62 ÷ 3.14 = d
d = 19.75
The diameter is 19.75 m.
C = 52 in
Find the radius (r) to the nearest
hundredth.
C=πxd
C=2xπxr
52 = 2 x 3.14 x r
52 = 6.28 x r
r = 52 ÷ 6.28
The radius (r) is 8.28 in.