1. Unit 4
Circle Terminology
Keystone Geometry

2. Circle Definition Circle :
The set of points coplanar points equidistant from a given point.
The given point is called the CENTER of the circle.
The distance from the center to the circle is called the RADIUS.
Center
Radius

3. Definitions Chord : The segment whose endpoints lie on the circle.
Diameter :
A chord that contains the center of the circle.
Tangent :
A line in the plane of the circle that intersects the circle in exactly one point.
Tangent
Point of Tangency :
Chord
The point where the tangent line intersects the circle.
Diameter
Secant :
A line that contains a chord.
Secant

4. Example: In the following figure identify the chords, radii, and diameters.B F C E
Radii:
Diameter:

5. Definitions Congruent Circles : Concentric circles :
Circles that have congruent radii. 2 2
Concentric circles :
Circles that lie in the same plane and have the same center.

6. Polygons A polygon inside the circle whose vertices lie on the circle.
Inscribed Polygon:
A polygon inside the circle whose vertices lie on the circle.
Circumscribed Polygon :
A polygon whose sides are tangent to a circle.

7. ARCS
Arcs :
The part or portion on the circle from some point B to C is called an arc. A B C
Example: B
Semicircle:
An arc that is equal to 180°. O A
Example: C

8. Minor Arc & Major Arc Minor Arc :
A minor arc is an arc that is less than 180°
A minor arc is named using its endpoints with an “arc” above. A
Example:
Major Arc:
A major arc is an arc that is greater than 180°. B B O
A major arc is named using its endpoints along with another point on the arc (in order). A
Example: C

9. Example: ARCS Minor Arc: Major Arc: Semicircle:
Identify a minor arc, a major arc, and a semicircle, given that is a diameter.
Minor Arc: A C D E F
Major Arc:
Semicircle: