Suppose a unit circle, and any arc S on the unit circle in the first quadrant. No matter where S is provided, the area between S and the x-axis plus the covered area between S and y-axis is constant! Moreover, that constant is same to the length of S:
A + B = s2 - s1.
In Figure, note that regions A and B overlap; in that part the area is counted double times. The quantity (s2 - s1) shows the length of S along the arc from s2 to s1.