Circles and Circumference

Lesson 9-5

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.

Identifying the Parts of a Circle

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

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

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.

Numerical Relationships A radius is exactly one-half of a diameter. Therefore a diameter is twice a radius.

5.5 cm

If the radius is 5.5 cm, then the diameter is ___________ cm.11

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.

Circumference Formulas

C = ¶dC = 2¶r

Example

41 m

C = ¶dC = (3.14)(41)

C = 128.74 m

We substitute 3.14 in for pi.

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