More about AlgebraLearning Objective: To use formulae to find the area and the circumference of a circle.

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This is the formula to find the circumference of a circle.

What does the ‘d’ stand for? What does the π mean?

c=πd

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The ‘d’ stands for diameter. The symbol π is called pi. Pi is a number that has trillions

of digits! Pi can be shortened to 3.14.

c=πddiameter

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What is the circumference of this circle?

The diameter is 5cm.

c=πd5cm

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Did you remember to use

3.14 as your value of π?

c=πd

5cm 15.7cm c=3.14 x 5

c=15.7cm

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What is the circumference of this circle?

The diameter is 18cm.

c=πd18cm

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Did you know most calculators have a button for

π?

c=πd

18cm

56.5cm c=3.14 x 18c=56.5cm

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What to you think this formula is for?

What does the ‘a’ stand for? What does the ‘r’ stand for?

a=πr2

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The ‘a’ stands for area. The ‘r’ stands for radius. The

radius is half of the diameter of a circle. This formula will

find the area of a circle.diameter

a=πr2

radius

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What is the area of this circle?

The radius is 8cm.

8cm

a=πr2

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Did you get the correct area?

201cm2 a=3.14 x 82a=πr2

8cm a=3.14 x 64a=201cm2

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What is the area of this circle?

The diameter is 24cm.

24cm

a=πr2

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Did you get the correct area?

452.2cm2 a=3.14 x 122a=πr2

a=3.14 x 144a=452.2cm2

24cm 12cm

This formula is for the circumference of a circle.

Can you find the perimeter of this semicircle?

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Plenary c=πd15cm

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p=½πd + dTo find the perimeter of the

semicircle you need to work out the circumference of the circle, divide it

by two and add the diameter.

diameter

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Did you get the correct

perimeter?p=½ (3.14 x 15) +15p=½ (47.1) +15

p=38.55cm

p=½πd + d

p=23.55 + 15

38.55cm

This formula is for the area of a circle. Can you find the

area of this semicircle?

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15cm

a=πr2

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a=½πr2To find the area of the semicircle you

need to work out the area of the circle and divide it by two.

diameter

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Did you get the correct area?a=½ (3.14 x 7.52)

a=½ (3.14 x 56.25)

a=88.3cm2

a=½πr2

a=½ (176.6)88.3cm2

15cm 7.5cm