PowerPoint Presentation - Keming Primary School

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					                        M6.1b


      Circles
Primary 6 Mathematics
                               Circles




Chapter Learning Outcomes

 Recognise   semi-circles &
 quadrants
Use  the formula to find
 circumference of circle




                                    2
                                                  Circles




Parts Of A Circle - Circumference
 Theperimeter of a circle is called the
 circumference.

 To measure the
 circumference of a circle or
 circular object:
     Wrap a length of string around it and
      use your finger or marker to indicate the
      end
     Place the string flat against a ruler to
      read the measurement

                                                       3
                                                                 Circles


   Parts Of A Circle – Circumference
   Activity 2
        Draw the circles with the given measurements,
         then complete the table.
        What do you notice about the values in the
          last column?
Circle    Diameter   Circumference   Circumference  Diameter
                                       (correct to 2 decimal places)
  A         2 cm
  B        3.5 cm
  C         5 cm


                                                                       4
                                                                Circles




      Parts Of A Circle - Circumference
Circle   Diameter   Circumference   Circumference  Diameter
                                      (correct to 2 decimal places)
  A       2 cm        ≈ 6.3 cm              ≈ 3.15
  B       3.5 cm      ≈ 11 cm               ≈ 3.14
  C       5 cm        ≈ 15.7 cm             ≈ 3.14

     What do you notice about the values in the
      last column?
     The circumference of any circle when divided
      by its diameter always has the same value.
                                                                      5
                                                Circles




Parts Of A Circle - π
   We usually take the value of π to be 3.14
    or 22 .
        7
   You can also use the value of π in your
    calculator.

 Therefore:


       Circumference = π  Diameter
      Circumference = 2  π  Radius
                                                     6
                                          Circles




Example
The diameter of a wall clock is 15 cm.
Find its circumference. (Take π = 3.14)


Circumference = π  Diameter
             = 3.14  15 cm
             = 47.1 cm




                                               7
                                            Circles




Example
The radius of a circular coaster is 7 cm.
                                   22
Find its circumference. (Take π= 7 )


Circumference = 2  π  Radius
                   22
             = 2  7  7 cm
             = 44 cm




                                                 8
                                   Circles




Example
A wheel has a diameter of 60 cm.
Find its circumference.
Give your answer correct to
2 decimal places.

Circumference = π  Diameter
             = π  60 cm
             ≈ 188.496 cm
              ≈ 188.50 cm
                                        9
                                    Circles




Semi-circles & Circumference
   A semi-circle is half a
    circle.




    Circumference of a         1
                                2
     semi-circle
         1
     = ( 2  Circumference of
     circle) + Diameter

                                        10
                                   Circles




Quadrants & Circumference
   A quadrant is a
    quarter of a circle.




   Circumference of a         1
                               4
    quadrant
        1
    = ( 4  Circumference of
    circle) + (2  Radius)

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posted:8/5/2012
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