The circumference is defined as pi multiplied by d, which is the diameter of the circle. This is equal to 2 multiplied by r multiplied by pi. Because the radius is 2.2 times the radius equals the diameter. So we can use either this or this to work out the circumference. let's look at an example. Let's say we have a circle with a radius of 5 centimeters. We can use this and this to say the circumference is equal to 2 multiplied by pi multiplied by 5. This, we can simplify down to 10 pi, because we have 2 multiplied by 5 multiplied by pi. Now pi is an irrational number. We can leave it in this form and this is correct. However, if you would like a shortened decimal answer, this is approximately 10 multiplied by 3.14. This gives us the answer 31.4 centimeters. Always remember your units.should have units here as well because this is a distance around the edge of the circle. Let's try one more example. Let's say we have a diameter of 20. So diameter is 20 centimeteres. Now we can use this to work out the circumference. It's very simple.we substitute the 20 in for the d here and we get 20 pi centimeters. If we want to simplify this once again, use 3.14 for pi and we get 20 multiplied by 3.14 which gives us 2 times 31.1, which gives us 60 plus 220.127.116.11 centimeters. And that is how to work out the circumference of a circle.