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Perimeter of a circle

The perimeter of a circle can be calculated with these two formulasp=2πrp=πd \begin{aligned} p &= 2 \pi r \\ p &= \pi d \end{aligned} with radius rr or diameter dd known.

If you want to find for example the radius of the circle but only know the perimeter, you can rearrange the formula for rr and get what you were looking for.