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Circles

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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Circles

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  1. Circles

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

  3. 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.

  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. 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

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

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

  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 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.

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