Question 475853
Assuming that a is the angle opposite vertex A, the proof is quite straightforward. The arc intercepted by angle a and the arc intercepted by angle c do not intersect, and they add up to the whole circle (360 degrees). This means that angle a + angle c = 180 degrees (since the angle with vertex on the circle is half of the degree measure of the arc). Similarly, angle b + angle d = 180 degrees, so we are done.