Circle – the set of all points in a plane a given distance away from a center

point.

A

Circle – the set of all points in a plane a given distance away from a center

point.

A

A circle is named by its center point. For example: Circle

A or A.

Circle – the set of all points in a plane a given distance away from a center

point.

A

A circle is named by its center point. For example: Circle

A or A.

Radius – the “given distance away from the center point” of a circle; a segment that joins the center to a

point on the circle.

Circle – the set of all points in a plane a given distance away from a center

point.

A

A circle is named by its center point. For example: Circle

A or A.

Radius – the “given distance away from the center point” of a circle; a segment that joins the center to a

point on the circle.

Radius (r) Plural: Radii

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

point.

Chord – a segment whose endpoints lie on on the circle.

Chord – a segment whose endpoints lie on on the circle.

Example: DC

C

D

Chord – a segment whose endpoints lie on on the circle.

Example: DC

A BC

D

Diameter – a chord that passes through the center of the circle. Example: AB

A diameter is twice the length of a radius.

Secant – a line that contains a chord.

Example: ABA

B

Secant – a line that contains a chord.

Example: ABA

B

**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

Tangent – a line that intersects a circle at exactly one point.

A

B

Example: AB

Tangent – a line that intersects a circle at exactly one point.

Not a tangent

!

A

B

Example: AB

Tangent – a line that intersects a circle at exactly one point.

Not a tangent

!

A

B

The point at which the circle and the tangent intersect is called the

point of tangency.

Example: AExample: AB

Congruent Circles – circles with

congruent radii.5cm

5cm

Congruent Circles – circles with

congruent radii.5cm

5cm

Concentric Circles – circles with the same center point.

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.

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.

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.

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

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

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.