The degree of the chord tangent angle is equal to half the degree of the arc it encloses.

The degree of the angle inside a circle is equal to half of the sum of the degrees of the arcs subtended by this angle.

The degree of the outer angle of a circle is equal to half the difference between the degrees of two arcs cut by this angle.

1. circumference c = 2π r = π d.

2. The area of the circle s = π r 2;

3. Sector arc length l=nπr/ 180