a3/q² + a3/q + a3 = S33/2(1/q² + 1/q + 1) = 9/21/q² + 1/q + 1 = 31/q² + 1/q - 2 = 0(1/q - 1)(1/q + 2) = 0q = 1 或 q = -1/2当 q = 1 时 ,a1 = 3/2当 q = -1/2 时 ,a1 = 6