Điền từ thích hợp vào chỗ trống

On Ox rays, take points A1, A2, ..., An, ... such that for each positive integer n, OAn = n. In the same half of the plane whose edge is the straight line containing Ox rays, draw half circles with diameter OAn, n = 1, 2,…. The symbol u1 is the area of a half circle with diameter OA1 and for each n ≥ 2, the symbol un is the area of the figure limited by a half circle of diameter OAn - 1, half circle of diameter OAn and Ox rays ( h 3.3). Prove that the sequence (un) is an arithmetic progression. Determine the right of that addition.