Round-the-island runway chasing formula: distance of A+distance of B = circular perimeter. The chasing and meeting problems involved in the movement of two objects on the same straight line or closed figure are usually classified as chasing problems. This kind is often found in exams. Generally, it can be divided into two types: one is that two people chase and two people meet, which is relatively simple; One is that it is more difficult for many people to catch up and meet.

In mathematics, a ring is a circular geometric figure, or more generally, a circular object. Geometrically speaking, a ring is a ring, and a large disk digs out the rest of a small concentric disk.