假设 a-2 = -3 ==> a = -1 则2a^2 + 5a = 2 * (-1)^2 + 5 * (-1) = 2 - 5 = -3 a - 2 = 2a^2 + 5a = -3 这与集合的定义不相符.所以 只有可能 2a^2 + 5a = -3.2a^2 + 5a = -3 ==> 2a^2 + 5a + 3 =0 ==> (2a + 3)(a +1) = 0 ==> a = -3/2 (a = -1 舍去,前面已经讨论过了.)