Since a circle is a special shape, you cannot find its area the way you would with a polygon. Also, a circle does not have a perimeter; it has what is called the circumference. The circumference is the measure around the outside of a circle. Since the circle is so different, its formulas are different too. The formulas for area and circumference both use a symbol to represent pi. The symbol or pi is . Its rounded value is 3.14 as a decimal and 22/7 as a fraction equivalent. When solving a problem with other fractions, the fraction would be used. The decimal form of pi can be used when working with whole numbers and decimals. The formulas for circles are as follows:
Circumference: C = d (This can be remembered by cd.)
Area: Circle: A = r 2 (This can be remembered by ar.)
In these formulas, d stands for diameter. The diameter of a circle is the measure from one edge of the circle to the opposite edge. The r stands for radius. The radius is the measure from the center of the circle to the outside edge. The radius of a circle is ½ the size of the diameter.