Types of Angles
1. central angle = arc
2. angle formed by radius and tangent = 90°
3. inscribed angle = 1/2 arc
4. angle formed by tangent and chord = 1/2 arc
5. angle formed by 2 lines intersecting inside a circle      
    = 1/2 of the sum of the 2 intercepted arcs
6. angle formed by 2 lines intersecting outside a circle      
    = 1/2 of the difference of the 2 arcs


Important postulates, definitions, and theorems
  1. All radii of a circle are congruent.
  2. If a radius is perpendicular to a chord, then it bisects it.   (converse is also true)
  3. If 2 arcs of a circle are congruent, then the chords are congruent.
    (converse is also true)
  4. If 2 chords are equidistant from the center of a circle, then they are congruent.
    (converse is also true)
  5. If a radius bisects a chord, then it bisects its arc.   (converse is also true)
  6. An angle inscribed in a semicircle is a right angle.
  7. Tangent segments to a circle are congruent.
  8. A radius is perpendicular to a tangent.
  9. If two chords intersect inside a circle, the products of their segments are equal.