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

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 = ¶d C = 2¶r

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

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