# Perimeter and Area of Circles

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This week we have been learning how to calculate the perimeter and area of circles and annuli (donut shapes) using ‘pi’. Remember that ‘pi’ is the ratio between the diameter and the circumference of a circle and is approximately equal to 3.14 or 22/7. Try this interactive at Illuminations, the Circle Tool.

# Circumference of a circle and Pi

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Today, 6/7G learnt about the ratio between the radius, diameter and circumference of a circle. We started with three lengths of string – 1m, 2m and 3m in length. Our task was to draw a circle, using half the length of the string as the radius of the circle, on the concrete with chalk. Then we used the string to measure how many times the string (diameter of the circle) went around the circumference.

Each group found that the circumference was ‘three and a bit’ times the diameter of the circle. More acurately, that ratio is ‘pi’ and is represented by the symbol below. Even thousands of years ago the Egyptians knew about this ratio, although they didn’t call it ‘pi’. The Babylonians, Indians, Greeks and Chinese mathematicians were fascinated by the irrational number, pi. You can read more about the magic of pi and Pi day. You may even like to celebrate Pi day next March (3rd month) 14th by making a ‘pi’ pie!