The document discusses several angle theorems: 1) The angle subtended by an arc at the center of a circle is double the angle subtended by the same arc at the circumference. This is proven using properties of isosceles triangles. 2) The angle in a semicircle is a right angle of 90 degrees. This is proven using the fact that the angle at the center of a full circle is 180 degrees, and the relationship that the central angle is double the circumference angle. 3) Some examples of exercises involving applying these angle theorems are provided.