Center Point, Radius, Central Angle
The center point in a circle is called just that: the center! The center point
is what is used to name the circle. The following circle would be called cir-
cle Cor C. The distance from the center of the circle to any point on the
edge of the circle is called the radiusof the circle. The radius is the defin-
ing element of a circle, determining how big it is. A circle has an infinite
number of radii, which are all the same length (“radii” is the plural of
“radius”). If two circles share the same center, but have radii of different
lengths, then the circles are concentric. Below, there are two concentric
circles, named C. The smaller one has radius CAand the larger one has a
radius CB. Notice that the two additional radii drawn into the smaller cir-
cle are equal in length to radius CA.
Chords and Diameters
A chordis a line segment that joins two points on a circle. On the fol-
lowing page, ACis a chord.
A diameteris a chord that joins two points on a circle and passes through
the center point. DBis a diameter of O.
Note: A diameter is twice the length of a radius, and a radius is half the
length of a diameter.
CAB501 Geometry Questions