You can put this solution on YOUR website! a circle has no vertex.
if you mean an angle whose vertex lies on the circumference of a circle, then you are talking about an inscribed angle.
a central angle has its vertex at the center of the circle.
the degree measure of a central angle is equal to the degree measure of its intercepted arc.
the degree measure of an inscribed angle is equal to 1/2 the degree measure of its intercepted arc.
the intercepted arc is the part of the circumference of the circle that is between the sides of the angle formed.
try this reference to learn more. http://www.mathopenref.com/circleinscribed.html
