CIRCLE

Circle

Definition. Circle is a set of all points in the plane which are equidistant from a given point О, called the center of circle.

Properties of a circle

1. Diameter of circle is equal two radiuses.

D = 2r

2. The shortest distance from the center circle to the secant (chord) is always smaller radius.

3. Three points that not placed on a straight line can hold only one circle.

4. Among all closed curves of equal length, circle has the largest area.

5. If two circle touch at one point, this point placed on the line that passes through the centers of the circles

Area and circumference of circle

Length of circumference

1. Formula of the circumference length in terms of the diameter:

C = πD

2. Formula of the circumference length in terms of the radius:

C = 2πr

Formula of the circle area

1. Formula of the circle area in terms of the radius:

A = πr²

2. Formula of the circle area in terms of the diameter:

A = πD²4