Area of a circle

The formula for working out the area of a circle is:

A = \pi r^2, where r is the radius of the circle.

\pi r^2 means \pi \times r \times r. Only the r is squared.

Question

Find the area of the following circles, using \pi = 3.14.

a) a circle of radius {6~cm}

b) a circle of diameter {10~cm}

a) r = 6~cm, so we calculate:

A = 3.14 \times 6 \times 6 = 113.04~cm^2

b) The diameter is {10~cm}, so the radius is {5~cm}. We calculate: A = 3.14 \times 5 \times 5 = 78.5~cm^2

Question
Dartboard

The dartboard above has a radius of {20~cm}. The bullseye in the centre of the board has a radius of {1~cm}.

By calculating the area of the two circles, work out the area of the dartboard outside of the bullseye, using \pi = 3.14.

Remember, the area of the large circle is {3.14}\times{20}\times{20} = {1,256}~cm^{2}.

The area of the small circle is 3.14 \times 1 \times 1 = 3.14~cm^2.

So, the area of the dartboard outside of the bullseye is {1,256} - {3.14} = {1,252.86}~cm^{2}.

curriculum-key-fact
When calculating the area of a circle, remember to use the radius, not the diameter.
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