In the third century BCE, Apollonius of Perga asked how many circles one could draw that would touch three given circles at exactly one point each. It would take 1,800 years to prove the answer: eight ...
The perimeter is the total distance around the edge of a 2D shape. Since it is a length, the metric units for perimeter are millimetres (mm), centimetres (cm), metres (m) and kilometres (km).
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