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central angles (vertex at the center of the circle) are equal to their encompassed arc
vertical angles (lines intersecting in a circle) are equal to the average of the encompassed arcs
a special case of the vertical angle is when the vertex is on the circumference of the circle
___ in this case ; one of the encompassed arcs is zero
___ the measure of the angle is still the average of the arcs ; but the average is half of the remaining arc