A cyclic quadrilateral has vertices on the same circle and is inscribed in the circle. The opposite angles have the same endpoints (the other vertices) and together their intercepted arcs include the entire circle. Since the measure of an inscribed angle is half the intercepted arc, the sum of the opposite angles must be 180 degrees.
Sample Problems (2)
Need help with "Cyclic Quadrilaterals and Parallel Lines in Circles" problems? Watch expert teachers solve similar problems to develop your skills.