# pi â€“ Ï€

pi is a number that goes on forever. So it has every number pattern possible somewhere inside it. It can be used to work out area and volume of circles and spheres.

Usually we can use pi with just 2 decimal places:

pi = 3.14

I have memorised it to 7 places as this is what a calculator shows:

pi = 3.1415927

My Dad memorised it to 21 decimal places when he was 10:

pi = 3.141592653589793238462

# Circles

Circumference of a circle:

**2Ï€r** (r is the radius which you can see from my picture below)

Diameter is 2 times the radius so the circumference can also be **Ï€d**

Area of a circle:

**Ï€r**^{2} (r^{2} means radius squared which means radius times itself)

*(this picture I drew using Inkscape but I had to save it as a png as the svg file could not be seen).*

# Spheres

Surface area of a sphere:

**4Ï€r**^{2}

Volume of a sphere:

**4/3Ï€r**^{3}

*(this picture was from wikipedia)*

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