41st Polish Mathematical Olympiad Problems 1990

41st Polish Mathematical Olympiad Problems 1990A1.  Find all real-valued functions f on the reals such that (x-y)f(x+y) - (x+y)f(x-y) = 4xy(x2-y2) for all x, y. A2.  For n > 1 and positive reals x1, x2, ... , xn, show that x12/(x12+x2x3) + x22/(x22+x3x4) + ... + xn2/(xn2+x1x2) ≤ n-1.