 There are two main formulas when dealing with circles. One of the formulas deals with circumference. Let's call it C. And the other deals with area. Let's call it A. The circumference formula, as perhaps you remember, is found with pi times diameter. And diameter, of course, is the distance across a circle. Another way to find circumference, by the way, since a diameter is 2 radii, you can take 2 times a radius times pi, which would give you the same thing. Now the area is slightly different and the difference is so slight that you'll have to be really careful to remember the difference. The area formula is pi times radius squared. Now these two formulas look very similar. Here I've got 2 radii times pi, but here we've got radius squared times pi. So, for example, if the radius was, oh, I don't know, let's say the radius is 5, then the area would be pi times 5 squared. In other words, it would be 25 pi. The circumference in this example would be 2 times 5 times pi, which would be 10 pi. So, again, keep your area and your circumference. Keep them straight.