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What is a circle?

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  • 1. www.thatmathlady.com © 2012WHAT 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 www.thatmathlady.com © 2012 curved line of the shape are equidistant from that point.  Equidistant means “having an equal distance”
  • 3. www.thatmathlady.com © 2012WE 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: www.thatmathlady.com © 2012
  • 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. www.thatmathlady.com © 2012 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 www.thatmathlady.com © 2012 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. www.thatmathlady.com © 2012 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!
  • 8. www.thatmathlady.com © 2012 sectorMORE CIRCLE PARTS…
  • 9. WHAT DID YOU LEARN ABOUT CIRCLES? www.thatmathlady.com © 2012