Lesson 17 โ€” Activity 2: Finding the Radius of a Circle


Getting Ready


Remember from the previous activity that radius is the distance from a point along the edge of the circle to the exact centre of the circle. Radius is half the value of the diameter.

In this activity, you will practise finding the radius of circles.


This image shows the radius of a circle.


There are two ways to find the radius of a circle.

1. If you are given the diameter of a circle, simply divide the diameter by 2.

For example, if you are told that the diameter of a circle is 26 hm and are asked to find the radius, you would divide 26 by 2.

26 รท 2 = 13 hm

The radius of a circle with a diameter of 26 hm is 13 hm.


2. If you are given the circumference of a circle, you must first rearrange the equation for circumference so that you are now finding the radius. The rearranged equation is:

r = C
     2ฯ€

For example, if a circle has a circumference of 24 cm, what is its radius?

r = C
     2ฯ€


r =   24

    2 x 3.14


r = 3.82 cm


The radius of the circle is 3.82 cm.


Here's another example. What if a circle had a circumference of 100 km?

r = C
     2ฯ€


r =    100

      2 x 3.14


r = 15.9 km


Try This:


Solve these problems. Use the formulas above and a calculator to solve.


If you know the diameter, you can use a formula to calculate the radius.

The buttons on Robert's shirt each have a diameter of 8 millimeters. What is each button's radius?




You can use the formula to find the radius of this sign.

A Do Not Enter sign had a circumference of 45 cm. What is the radius of the sign?


For the button problem:


8 รท 2 = 4 mm
Each button has a radius of 4 mm.



For the sign problem:
r = C
     2ฯ€

r =   45

    2 x 3.14


r = 7.17 cm

The radius of the sign is 7.17 cm.



Images courtesy of www.imagesgoogle.com