Let me tell you a secret!
You know how the formula for Circumference is
C = 2πr
Where r is the radius?
The reason why this formula works the way it does is because of what π IS.
π is the ratio of ANY circle's Circumference to it's Diameter.
So you can think of π like this:
Circumference / Diameter.
2r is the same as the diameter. So what you are doing is canceling out the diameter. Similar to what you do in algebra I to solve for x. Then you are just left with the circumference. That's pretty neat!
So, let's get to it!
2πr
Sadly, you'll most likely need to use a calculator for this problem, but if you are not permitted one then replace π with 3.14.
What is r?
r = diameter / 2
r = 12 / 2
r = 6
2π6
If you are allowed to use a calculator then plug it in to it. If not then start doing some long multiplication!
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