Circles
Honors Math – Grade 4
Circle
A circle is a plane figure.
All the points on a circle
are the same distance
from a given point, called
the center.
Point A is the center of
the circle
A
Circle A
A circle is named by its center point.
The Parts of Circle B
All the line
segments in
Circle B have
special
names.
Can you
name any
points on the
circle?
E, C, D, F, G
C
E
B
D
G
F
Radius
 A radius is a line
segment with
endpoints at the
center of the circle
and on the circle.
 BE is a radius.
 Can you name any
other radii (plural of
radius)?
C
E
B
D
G
BD
BC
F
Diameter
C
E
B
D
G
F
12 cm
If BE measures 6 cm, how long is CD?
 A diameter is a line
segment that passes
through the center of
the circle and has
both endpoints on the
circle.
 CD is a diameter.
The length of the
diameter is always equal
to twice the length of the
radius.
Chord
C
E
B
D
G
F
 A chord is a line
segment with both
endpoints on the
circle.
 FG is a chord.
 Are there any other
chords in Circle B?
 Line segment CD
A diameter is a special
chord.
Simple Closed Curve
A simple closed curve is a path that begins
and ends at the same point and does not
cross itself.
Simple Closed
Curves
Not simple
closed curves
Practice
P
T
V
R
X
S
W
Q
1. Name the circle and
its center.
2. How many diameters
are shown? Name
the diameters.
3. Is TR a radius?
Explain why or why
not?
4. Is VX a radius?
Explain why or why
not?
5. How many radii are
shown? Name the
radii.
Practice Answers
1. ____________________________________________
2. ____________________________________________
3. ____________________________________________
4. ____________________________________________
5. ____________________________________________
Practice Answers
1. Circle V. The center is point V.
2. There are 3 diameters. PQ, RS, TW
3. TS is not a radius because it does not
have an endpoint at the center of the
circle and it is a chord.
4. VX is a radius because its endpoints are
the center point V and point X on the
circle.
5. There are 7 radii. VP, VS, VW, VQ, VX,
VR, VT