1. Introduction to Circles
Agenda:
Warmup-Cumulative Review
Circles Vocab

2. Unit 9
Circles

3. A circle is named by its center point. For example: Circle A or A.
Circle Vocabulary
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 example: Circle A or A. A
Sphere – the set of all points a given distance away from a center point.

4. Circle – the set of all points in a plane a given distance away from a center point.
Example: QA
Radius Q
Plural: Radii
Radius – a segment with one endpoint at the center of the circle (or sphere) and one endpoint on the circle (or sphere).
the “given distance away from the center point” of a circle.
Day 1

5. Diameter – a chord that contains the center of the circle. Example: AB
Chord – a segment whose endpoints lie on two different points on the circle.
Example: DC A B C
Day 1 D
Diameter – a chord that contains the center of the circle. Example: AB
A diameter is twice the length of a radius.

6. Secant – a line that contains a chord.
Example: AB B 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
Day 1

7. Tangent – a line that intersects a circle at exactly one point.
Not a tangent! B
The point at which the circle and the tangent intersect is called the point of tangency.
Example: A A
Example: AB
Day 1

8. Congruent Circles – circles with congruent radii.
5cm
Concentric Circles – circles with the same center point.
Day 1

9. This pentagon is inscribed inside of 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.

10. This circle is inscribed inside of the pentagon.
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.