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WHAT IS A CIRCLE?
By That Math Lady

2. THE DEFINITION
 A circle is a round, two-dimensional shape. It has a
middle point (center). All of the points that make the
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curved line of the shape are equidistant from that
point.
 Equidistant means “having an equal distance”

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WE SEE CIRCLES EVERYDAY…
…when we tell
time!
…when we use
money!
…when we go for
a ride!
…when we go on
a vacation!

4. HOW DO YOU DRAW A CIRCLE?
 Use a math compass:
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5. LINES INSIDE A CIRCLE
 There are many lines inside a circle.
 Any line that touches two points on the circle is called a
chord.
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 Any line that touches two points on the circle and goes
directly through the center is called a diameter.
 Any line that begins at the center and touches one point
on the circle is the radius. The radius is one-half the
length of the diameter.
radius
diameter
chord

6. CIRCLE MEASUREMENTS: CIRCUMFERENCE
 We use the diameter and radius to measure the area
and circumference of a circle.
 The circumference is the distance around a circle. (You
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may think of this as the circle’s perimeter.)
 To measure the circumference, multiply the diameter by
the number, π (Pi).
 π is equal to 3.14159265... We commonly use 3.14 to find an
estimate answer.
Example:
circumference = dπ
dπ = 2(3.14)
circumference = 6.28 cm

7. CIRCLE MEASUREMENTS: AREA
 To find the area of a circle, you must know the
radius or the diameter of the circle.
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 If you know the diameter, divide that number by 2
(this will give you the radius).
 Area = πr²
Example:
area = πr²
πr² = (3.14)(3)²
area = 3.14(9)
Radius = 3 ft area = 28.26 ft²
There are 28.26
squares inside this
circle!

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sector
MORE CIRCLE PARTS…

9. WHAT DID YOU LEARN ABOUT CIRCLES?
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