Circles: Basic Terms

Tutorial 8aTutorial 8a

Parts of a Circle

A circle is the set of all points in a plane equidistant from a given point.

Name a circle by its center:

P

The center of the circle below is point P.

Therefore the name of this circle is Circle P.Circle P.

Parts of a Circle

A radius is a segment that has one endpoint at the center and the other endpoint on the circle. Radius plural is radii.

P

Circle PCircle P

C

A

B

The following segments are radii of Circle p.

PC, PB, PAPC, PB, PA

Parts of a Circle

A chord is a segment whose endpoints lie on the circle.

P

Circle PCircle P

C

A

A diameter is a chord that contains the center of the circle.

A diameter is twice as long as the radius.

is a diameter of Circle P.BCBCB

BC = 2BC = 2•(BP)•(BP)

D

is a chord of Circle P.ADAD

Parts of a Circle

A secant is a line that contains a chord of a circle.

P

Circle PCircle PA

A tangent is a line in the same plane as a circle that intersects the circle in exactly one point.

The point of tangency is the point of intersection.

is tangent to Circle P.GEGE

D

is a secant of Circle P.ADAD

F•

•E

G•