You can't technically render arcs or perfect circles, you split your arc / circle / sector up into as many triangles or straight lines as you require to get your desired level of detail.
For a perfect circle and circular sectors <= 180 degrees, a convex n-gon portion will work, but circular sectors with an angle between 180 and 360 degrees could only be represented by a concave polygon, which is why you'll probably have to go the multi-triangle way.