Let S be the sum of the first n terms of the arithmetic sequence b1, b2, .....bn, such that f(i,j,k)= S_{i}/i * (j - k) + S_{j}/j * (k - i) + S k k (i-j) forall i,j,kЄ N, then
(1) (1, 3, 5) = 15
(3) f(1, 3, 5) = 21
(2) f(2, 7, 12)<21
(4) f(2, 7, 12) < 15
