当 b > 0 时, a + b = 5/2 a - b = -3/2 解得： a = 1/2 b = 2 当 b < 0 时, a + b = -3/2 a - b = 5/2 解得： a = 1/2 b = -2 所以 a = 1/2 ,b = 2 或 a = 1/2 ,b = -2
