CIRCLES—FOUR FORMULAS
After the triangle, the test makers’ favorite plane geometry figure is the circle.
Circles don’t come in as many varieties as triangles do. In fact, all circles are
similar—they’re all the same shape. The only difference among them is size.
So you don’t have to learn to recognize types or remember names. All you
have to know about circles is how to find four things:
You could think of the task as one of memorizing four formulas, but you’ll be
better off in the end if you have some idea of where the arc and sector
formulas come from and how they are related to the circumference and area
formulas.
Circumference: Circumference is a measurement of length. You could think
of it as the perimeter: It’s the total distance around the circle. If the radius of
the circle is r,
Circumference = 2πrSince the diameter is twice the radius, you can easily express the formula in
terms of the diameter d:
Sum of Exterior Angles = 360°Circumference
Length of an arc
Area
Area of a sector