1. Circles

2. Fold unfold A B AB divides the circle into equal halves.
AB is a diameter of the circle

3. A B O 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.

4. 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

5. Perimeter of circle A B
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

6. Perimeter of semi-circled
Circumference of a circle =  d
Perimeter of a semi-circle =   Radius + Diameter
= r + d

7. Perimeter of quarter-circle
Circumference of a circle =  d
Perimeter of a semi-circle =   Radius + Diameter
= r + d
Perimeter of quadrant = r + 2r

8. 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

9. Area of circle
Cut the circle into 24 equal pieces.

10. Area of circle
Rearrange 23 pieces like this.

11. Area of circle Area of circle =   r  r
Cut the last piece into halves and place one half at each end. The pattern looks like a rectangle.
Area of circle =   r  r

12. Shapes with the same area
The figure below is formed by a semicircle and two quarter circle.

13. Shapes with the same area
Area of the figure = 20 cm  10 cm
= 200 cm2

14. Shapes with the same area
ACDF is a rectangle made up of 2 squares.

15. Shapes with the same area
The figure shows semi-circles of equal areas at the sides of a square.

16. Shapes with the same area
Four identical circles of diameter 10 cm are arranged in a big circle.