A circle is the set of all points equidistant from a fixed point.
In short, a circle is round curve. The point equidistant from any point on the circle is center. It is the most important point in the circle. The distance between the center point and curve of the circle is called radius. Twice of radius is the diameter.
Circumference and Area of Circle
These formulae gives circumference and area of the circle. Also remember that diameter is two times the radius of the circle.
Chord of Circle
Any line segment that has both endpoints on the circle is a chord. Chord that runs through the center of the circle is diameter. The diameter is the longest possible chord.
Arc of Circle
The complete circle has angle of 360 degrees and corresponding length is called circumference of the circle. There is direct relationship between the length of the arc and the angle made by the circle.
L/(2πr) = θ/(2π)
L = θ r
Tangent to a Circle
A tangent line to a circle touches the circle at only one point. The radius of the circle is perpendicular to the tangent line at the point of tangency. Two tangents to a circle from a common exterior point of the circle are equal in length.