A illustration of the properties of the angles drawn from the ends of a chord in a circle. Circle Theorems http://www.geogebratube.org/student/c89/m1462/yryy

Two important results are the equality of the angles at the circumference and their connection to the angle at the centre. A special case that you are often expected to know is the result when the angle at the centre is 180. When AB becomes a diameter and you have a semi-circle. What is significant about the angle at the circumference in this case?