Lesson 17 — Activity 3: Finding the Diameter of a Circle
Lesson 17 — Activity 3: Finding the Diameter of a Circle
Getting Ready
In this last activity on circles, you will practise calculating the diameter of circles.
Remember that diameter is the distance from a point along one edge of the circle, through the centre of the circle, to a point on the opposite side of the circle.
There are two ways to find the diameter of a circle.
1. If you have been given the radius of the circle, to find the diameter, simply multiply the radius by 2.
For example, if you are told that the radius of a circle is 14 mm and are asked to find the diameter, you would multiply 14 by 2.
14 x 2 = 28 mm
The diameter of a circle with a radius of 14 mm is 28 mm.
2. The other way you can find the diameter of a circle is when you are given the circumference of a circle. To solve this you must first rearrange the equation for circumference so that you are now finding the diameter. The rearranged equation is:
d = C
π
For example, if a circle has a circumference of 36 cm, what is its diameter?
d = C
π
d = 36
3.14
d = 11.46 cm
The circle has a diameter of 11.46 cm.
Here's another example:
What if a circle had a circumference of 200 m?
d = C
π
d = 200
3.14
d = 63.69 m
Try This:
Using the formulas above, solve these problems to find the diameter. You may use a calculator to solve.
1. Sari's coffee mug has a radius of 3 centimetres. What is the mug's diameter?
2. A visitor to a cathedral measured the stained glass window and calculated that it has a circumference of 18.84 metres. What is the window's diameter?
1. 3 x 2 = 6 cm
The mug's diameter is 6 cm.
2. d = C
π
d = 18.84
3.14
d = 6 m
The window's diameter is 6 m.