1. Circles and Circumference
Section 10.1
Geometry

2. Terms
Circle – set of all ordered points that are equidistant from a given point (center), in a plane.
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.

3. Identify Parts of a Circle
Name the circle
Circle A
Name a radius
Name 2 chords
Name a diameter

4. Important Theorems/Formulas
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

5. Find measures in intersecting circles
The diameters of Circle A, Circle B, and Circle 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
BY = 3

6. What is Circumference?
Circumference – is the distance around the circle.
Circumference is 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.)
If given the diameter: C =
If given the radius: C =

7. Example Problems: Find C if r = 13 inches. C = Find C if d = 6 mm
Find d and r if C =
d = 34
If d = 34 then r = 17