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12WHAT IS A CIRCLE?
By That Math Lady
ww
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THE DEFINITION
A circle is a round, two-dimensional shape. It has a middle point (center). All of the points that make the curved line of the shape are equidistant from that point. Equidistant means “having an equal distance”
WE SEE CIRCLES EVERYDAY…
…when we use money!
…when we tell time!
…when we go on a vacation!
…when we go for a ride!
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ww
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HOW DO YOU DRAW A CIRCLE?
Use a math compass:
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LINES INSIDE A CIRCLE There are many lines inside a circle. Any line that touches two points on the circle is called
a chord. 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
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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 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. di
amet
er =
2 cm
Example: circumference = dπ
dπ = 2(3.14)circumference = 6.28
cm
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CIRCLE MEASUREMENTS: AREA
To find the area of a circle, you must know the radius or the diameter of the circle.
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)²area = 3.14(9)
area = 28.26 ft²Radius = 3 ft
There are 28.26 squares inside this circle!
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MORE CIRCLE PARTS…
tangent lin
e
arc
sector
segm
e
nt
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WHAT DID YOU LEARN ABOUT CIRCLES?
