1. Section 9-1
Basic Terms

2. Circle
The set of points in a plane at a given distance (radius) from a given point (center) in that plane.
Center
Radius

3. Radius Any segment that joins the center to a point of the circle
All radii of a circle are congruent!

4. Chord Secant A segment whose endpoints lie on a circle
A line that contains a chord
It goes through a circle!

5. Diameter
A chord that contains the center of a circle

6. tangent
A line in the plane of a circle that intersects the circle in exactly one point, called the point of tangency.

7. Circle P
Chord
Diameter P
Secant

8. sphere
Set of all points in space at a given distance from a given point.

9. Many of the terms used with circles are used with spheres:

10. Congruent Circles (or spheres)
Circles (or spheres) that have congruent radii.

11. Concentric circles
Circles that lie in the same plane and have the same center

12. Concentric spheres
Spheres that have the same center

13. A polygon is inscribed in a circle and the circle is circumscribed about the polygon when each vertex of the polygon lies on the circle.
Example: