1. Circles and Circumference
Lesson 9-5

2. Vocabulary
A circle is a plane figure that consists of a set of points that are equidistant from a given point called the center.
The circumference of a circle is the distance around it.

3. Identifying the Parts of a Circle
A radius is a line segment that connects the outside of the circle to its center.

4. A diameter is a line segment with both endpoints on the circle that passes through the center.

5. A chord is a line segment whose endpoints are on the circle.
Chords do not have to pass through the center of the circle. However, if a chord does pass through the center, it is also considered a diameter.

6. Numerical Relationships
A radius is exactly one-half of a diameter.
Therefore a diameter is twice a radius.
If the radius is 5.5 cm, then the diameter is ___________ cm. 11
5.5 cm

7. Circumference
Remember, circumference is the distance around the circle.
If you divide a circle’s circumference by its diameter, you always get the same irrational number – pi (symbol: ¶ )
This is true of every circle.
We estimate pi to be 3.14 or the fraction 22/7.

8. Circumference Formulas
C = ¶d
C = 2¶r

9. C = ¶d C = (3.14)(41) C = 128.74 m Example 41 m
We substitute 3.14 in for pi.

10. Homework Time