Circumference
We use 
with formulas to find the circumference of circles. For example, if you are given the diameter of a circle, you can find the circumference by using this formula:
Another formula for finding the circumference of a circle is:
As you can see, a diameter is equal to 2 radii. So, if you are given the measure of the diameter, use the formula, C = x d. If you are given the measure of the radius, use the formula, C = 2 x x r.
Try these now. (Remember is approximately equal to 3.l4, 3.l4l6 or 3 l/7.)
Are your answers reasonable? Recall that the circumference of a circle is about three times the measure of the diameter. In our first example the diameter was l2 inches. 3 x l2 = 36 and our answer for the example was 37.68 inches. 36 is a reasonable estimate.
If you want a more precise answer for your problem, use = 3.1416.
Example:
If you are working with a fractional number for the diameter you can use 3 1/7 for .
Example:
