Circles and Circumference

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.

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 Formulas

C = πdC = 2πr

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: π )• This is true of every circle.•We estimate pi to be 3.14 or the fraction 22/7.

Example

41 m

C = πdC = (3)(41)

C = 123 m

We substitute 3 in for pi.

Example

6in

C = 2πrC = (2)(3)(6)C = 36 in

We substitute 3 in for pi.