Circles and Circumference
Section 10.1
Circle – set of all ordered points that are
equidistant from a given point (center).
Chord – segment whose endpoints are
points on the circle.
Radius – segment with endpoints that are
the center and a point of the circle
Diameter – a chord that passes through
the center of the circle
Identify Parts of a Circle
Name the circle
Circle A or A
 Name a radius
B
C
F
AB AC AD AE AF
A
Name 2 chords
D
BC FD
Name a diameter
FD
E
All radii in a circle are congruent.
A diameter (d) is composed of 2 radii (r).
So all diameters in a circle are congruent.
d = 2r
Find measures in intersecting circles
 The diameters of A, B, C are 10 inches, 20 inches,
and 14 inches respectively. Find XB.

d = 10 in circle A, AX = 5

d = 20 in circle B AB = 10 and BC = 10

AX + XB = AB

5 + XB = 10

XB = 5

Find BY

d = 14 in circle C, YC = 7

BY + YC = BC

BY + 7 = 10
Circumference – is the distance around
the circle.
Circumference is most represented by C
The ratio of the circumference of a circle
and the diameter of the circle is an
irrational number called pi. The Greek
letter  is used as the exact value.
C =  d
Or C = 2 r
Find C if r = 13 inches.
C = 2 13   26
Find C if d = 6 mm
C =   6   6
Find d and r if C = 34
34   d
d = 34
If d = 34 then r = 17