But I know what they are

1. Circles But I know what they are

2. Parts of a Circle Radius Diameter OK, I’ve got all that, but what is all that stuff about Pi and how does it help with circles? Circumference Chord

3. Pi (mmm pie) Pi may seem daunting, but it really is easy as Pi. Basically Pi equals 3.14* and is used to calculate the circumference, diameter and radius of a circle. It has a special symbol – as shown on your calculator… * This is only what Pi is most commonly known as. It actually has 100,000 decimal places! Click the link to find out what they all are!

4. Formulae (or Formulas) - Circumference The formula for working out the circumference of a circle is: C= 2πr It means that the circumference (C) is equal to 2 times pi multiplied by the radius. That’s right! You can also say C= πd (pi times the diameter), because the radius is half the diameter.

5. Formulae (or Formulas) – Diameter & Radius If you’re given the circumference of a circle and have to work out the diameter or radius the formula is exactly the same. For example, the circumference was 16 cm the formula would be: 16=2πr

6. Area of a Circle Area of the circle also uses Pi in its formula: A=πr2. Basically this means the area of a circle is equal to Pi multiplied by 2 and multiplied by the diameter Uh-huh, I think I’ve got it! But wait, maths is not just about the formula, it’s about the actual sum – how do I know how to do that right?

7. Practice makes Perfect… Try these sums and see if you get the right answer…. Good Luck!

8. The radius of a circle is 8 cm, but what is the circumference? C=2πr C= 2 x π x r C= 2 x π x 8 2 x π = 6.28 6.28 x 8 = 50.24 cm

9. And now see if you can work this one out – If Jenny’s bicycle wheel is 70 cm in perimeter, what is its diameter 70 = 2πr 70 = 2 x π x R R= 70/2x π 70= 109.9 I can do this! How about some questions on area – I’m ready for a challenge!

10. Here are some Area questions for you to try if you feel you are ready.... Amy measures the diameter of her circular kitchen table as 90 cm. She uses π to work out its area. A= πr2 A= π x 452 A=6358.5 cm

11. Pi seems quite easy… If you think that, we’d better do some harder questions!

12. Find the Circumference A plate has a radius of 15cm. Find the circumference. Nooooo!!!! These will be really REALLY hard! Using C = π d d = 2 x r d = 2 × 15 = 30cm C = π × 30 C = 94.2cm

13. Click here to play Pi games Wait! That was actually easy... Can I do another one? Find the area of a circle with a diameter of 20cm. A= π r2 A = π × 102 = 314.2cm2 A = 314.2cm2

14. Pi Page – Irrelevant Facts • The symbol for pi (π) has been used regularly in its mathematical sense only for the past 250 years • We can never truly measure the circumference or the area of a circle because we can never truly know the value of pi. Pi is an irrational number, meaning its digits go on forever in a seemingly random sequence • If the circumference of the earth were calculated using π to only the ninth decimal place. It would miss out in 25,000 miles around the earth • In 1995, Hiroyoki Gotu memorized 42,195 places of pi and is considered the current pi champion. • Pi has been studied by the human race for almost 4,000 years.