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Circles are amazing shapes. Every single point on a circle is the same distance from the centre. Also they are heavily involved with the irrational number which we call pi. Pi has a never-ending decimal tail that never shows a pattern.
The diameter fits around the outside 3 and a bit times (π):
Or, if you want to use the diameter:
There will be a π button on your calculator.
Again, pi is involved in the area formula: