A circle may also be defined algebraically, as the set of solutions to an equation of the form:
- (x − a)2 + (y − b)2 = r2,
where (a,b) is the center of the circle.
The distance around the circle, or circumference, is given by:
- C = 2π * r
The area inside the circle is calculated using the formula:
- A = π * r2
A circle is a conic section, the intersection of a plane with a cone such that the plane is perpendicular to the axis of the cone.
Circles can readily be constructed by using a fixed distance between a pencil point and the center. A compass is a tool for doing this easily.
A circle with r = 1 is called the unit circle, and is used extensively in trigonometry.