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Circles - PowerPoint PPT Presentation


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Circles. unfold. A. B. Fold. AB divides the circle into equal halves. AB is a diameter of the circle. O. A. B. C. Fold the paper circle in half in another way to obtain another diameter of the circle. The two lines meet at point O. O is the centre of the circle.

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Presentation Transcript
slide2

unfold

A

B

Fold

AB divides the circle into equal halves.

AB is a diameter of the circle

slide3

O

A

B

C

Fold the paper circle in half in another way to obtain another diameter of the circle.

The two lines meet at point O.

O is the centre of the circle.

The line OC is the radius of the circle.

definitions
Definitions
  • The diameters of a circle pass through the centre of the circle.
  • The diameter of a circle is twice its radius
  • Diameter = 2 Radius
  • Radius = Diameter 2
perimeter of circle

A

B

Perimeter of circle

The circumference of a circle is slightly longer than 3 times its diameter.

d

This approximate value, 3.14 is represented by . We read  as ‘pi’.

Circumference of a circle =   Diameter

= d

perimeter of semi circle
Perimeter of semi-circle

d

Circumference of a circle = d

Perimeter of a semi-circle =   Radius + Diameter

= r + d

perimeter of quarter circle

r

Perimeter of quadrant = r + 2r

Perimeter of quarter-circle

Circumference of a circle = d

Perimeter of a semi-circle =   Radius + Diameter

= r + d

area of circle
Area of circle

A circle is drawn on a 1-cm square grid.

Radius of circle = 10 cm

Area of quarter circle = 76 cm2

Area of circle = 4 x 72

= 304 cm2

area of circle1
Area of circle

Cut the circle into 24 equal pieces.

area of circle2
Area of circle

Rearrange 23 pieces like this.

area of circle3
Area of circle

Cut the last piece into halves and place one half at each end. The pattern looks like a rectangle.

Area of circle =   r  r

shapes with the same area
Shapes with the same area

The figure below is formed by a semicircle and two quarter circle.

shapes with the same area1
Shapes with the same area

Area of the figure = 20 cm  10 cm

= 200 cm2

shapes with the same area2
Shapes with the same area

ACDF is a rectangle made up of 2 squares.

shapes with the same area3
Shapes with the same area

The figure shows semi-circles of equal areas at the sides of a square.

shapes with the same area4
Shapes with the same area

Four identical circles of diameter 10 cm are arranged in a big circle.