Circles and Circumference

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 circles 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 2r
9
Example
C d
C (3.14)(41)
41 m
C 128.74 m
We substitute 3.14 in for pi.
10
Homework Time