Lesson 17 โ Activity 2: Finding Diameter
Completion requirements
Lesson 17 โ Activity 2: Finding Diameter
In the last activity, you learned how to find the circumference (the distance around the circle). Sometimes, you will need to find the diameter of the circle instead. Remember, diameter is the distance across the circle.
To find the diameter of a circle, use the following formulas:
If you know the radius, the formula is:
(where "r" is the radius of the circle multiplied by 2)
Remember, the radius is the distance from the centre of a circle to any point on the circle.
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If you know the circumference, the formula is:
(where "C" is the circumference of the circle divided by pi, or 3.14)
If you know the radius, the formula is:
Remember, the radius is the distance from the centre of a circle to any point on the circle.
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If you know the circumference, the formula is:
(where "C" is the circumference of the circle divided by pi, or 3.14)
Let's try some examples!
Using your calculator, find the diameter of the circle below.
You know that the radius (the distance from the centre of the circle) is 9 m,
therefore, use the formula:
d = 2r
d = 2 x 9
d = 18 m
d = 2 x 9
d = 18 m
Let's try another one. This time, find the diameter of the circle when you know the circumference.
The formula is:
Therefore:
d =
d = 3.18
Self-check!
Try This!
You will need to use your calculator to find the answers to the following questions.
Find the diameter of the following circle.
d = 2r
d = 2 x 12 in
d = 24 in
Find the diameter of the following circle.
Round your answer to the nearest tenths place.
d = c รท ฯ
d = 33.929 รท 3.14
d = 10.8 cm (rounded to the nearest tenth)