How to calculate the circumference (the outside) of a circle Now we work it out First you have to get pie! Next you need only one more number - the diameter

Easy for you guys Test Now you know how to do it all. Let's see if you paid attention.

No, not that kind of pie. We want Pi, which is a number calculated by dividing the circumference of a cirle by the diameter of that same circle. Luckily Pi is always the same, no matter what size the circle is. So if you remember Pi is 3.14 you'll never have to work it out! Pi is actually the name of a letter of the Greek alphabet. It is usually written in maths as the Greek letter, and it looks like this: Just to be safe, let's refresh our memory on the three common measurements to do with circles.

They are the Radius, Diameter, and Circumference.

Here's an easy way to remember which is which. RADIUS - Imagine you are the centre of a circle. If you raise one arm you are showing the radius - the straight line (in any direction) from the centre of the circle to the widest point on its edge. DIAMETER - Imagine you are the centre of a circle. If you raise both (double) your arms you are showing the diameter - the straight line (in any direction) from the edge of the circle at its widest point, through the centre, and on to the widest point on its opposite edge. CIRCUMFERENCE - Imagine you are the centre of a circle. If you raise both (double) your arms and turn 360 degrees (circle once) you are showing the complete outside edge, the circumference of the circle.

It might be useful to circle 314 times. Maybe not, because I don't want to clean up afterwards. Yet 314 times would help you remember pi. So... RAISE one arm for RADIUS

DOUBLE for DIAMETER

and CIRCLE for CIRCUMFERENCE If we know the radius, the diameter = radius x 2

If we know the diameter, the radius = diameter ÷ 2

If we know the diameter, the circumference = diameter x pi

If we know the circumference, the diameter = circumference ÷ pi

In each case, work out the two missing measurements of a circle.

1. The radius is 25cm

2. The radius is 48mm

3. The diameter is 96m

4. The diameter is 1km

5. The circumference is 314cm

6. The circumference is 37.68mm

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