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'Running Race' printed from https://nrich.maths.org/
Imagine a circle of diameter $1$ unit, and another circle of a
similar diameter, and another.
What is the total perimeter of these three circles (you can leave
your answer with $\pi$ in it)?
What is the perimeter of a circle of diameter $3$ units?
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Now imagine two circles - one of diameter $2$ units and one of
diameter $3$ units.
What is the total perimeter of these two circles?
What is the perimeter of a circle of diameter 5units?
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What do you notice?
Is this generally the case with combinations of circles whose total
diameters are equal?
Can you explain why?