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The formula of the circumference is  c=2πr1. first, find the length of the diameter and the length of the (_circumference_) of the circle.  2. next, divide the length of the circumference by the length of the (_diameter_) and set up an equation where the ratio equals ​ π .  3. and 4. then, solve the equation for the circumference.  finally, substitute 2 times the (_radius_)  for the (here's the options diameter radius arc area circumference volume( i think you need to add more details so i can you .. don't hesitate to reply or to add more details. i hope this would .)to explain it more pi or (π)  is the circumference of any circle divided by its diameter.  so,  π = c/dfrom this formula π = c/d, we can get the value of c = πd. and since d which is the diameter is twice its radius, therefore we have c=2πr.