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Key
Idea
A circle is a familiar
plane figure that is
not a polygon. |
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Vocabulary
• circle
• center
• radius
• diameter
• chord
• central angle |
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Materials
• compass
• ruler or
straightedge
• protractor |
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Segments and Angles
Related to Circles |
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| What are the names of segments and
angles related to a circle? |
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A circle
is a closed plane figure made up of all the points
the same distance from the center. A circle is
named by its center.
Circle O
is shown at the right. |
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A radius
(plural: radii) is any line segment that
connects the center to a point on the circle.
is a radius. |
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A diameter is any line segment through the center that connects two points on the circle.  is a diameter. |
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A chord is any line segment that connects two points on the circle.
 is a chord. |
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A central
angle is an angle whose vertex is the center.
 is a central angle. |
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What are some special relationships? |
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| a. |
Follow the directions at the
right to construct a circle with
a radius equal in length to the
length of  .
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| b. |
Use a ruler to draw a diameter. Compare its length to that of the radius. |
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| c. |
Draw another diameter. Measure the four central angles. What is the sum of the measures? |
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| d. |
Draw a third diameter. Measure the six central angles. What is the sum of the measures? |
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| e. |
Repeat steps a–d using a different radius. |
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