Section 9.1 - All vocabulary relating to circles.

3. Circle – the set of all points in a plane a
given distance away from a center point.
A circle is named by
its center point. For
A example: Circle A or
A.
Radius (r) Plural: Radii
Radius – the “given distance away from
the center point” of a circle; a segment
that joins the center to a point on the
circle.

4. Sphere – the set of all points a given
distance away from a center point.

5. Chord – a segment whose endpoints lie on
on the circle.
Example: DC
A B
C
D
Diameter – a chord that passes through
the center of the circle. Example: AB
A diameter is twice the length of a radius.

6. Secant – a line that contains a chord.
B Example: AB
A
**Note: A chord and a secant can be named
using the same letters. The notation tells you
whether it is a secant or a chord. A secant is
a line; a chord is a segment.**
Secant: AB Chord: AB

7. Tangent – a line that intersects a
circle at exactly one point.
Not a
B tangent!
A The point at which the
circle and the tangent
intersect is called the
point of tangency.
Example: AB
Example: A

8. 5cm Congruent Circles –
circles with
5cm congruent radii.
Concentric
Circles – circles
with the same
center point.

9. A polygon is inscribed in a circle
and the circle is circumscribed
about the polygon when all of
the vertices of the polygon lie
on the circle.
This pentagon is
inscribed inside
of the circle.

10. When each side of a polygon is
tangent to a circle, the circle is
said to be inscribed in the
polygon.
This circle is
inscribed
inside of the
pentagon.