Three unequal real numbers form an arithmetic sequence. After properly exchanging the positions of the three numbers, they become an arithmetic sequence. Then the common ratio of the arithmetic sequence is___ ．

Three unequal real numbers form an arithmetic sequence. After properly exchanging the positions of the three numbers, they become an arithmetic sequence. Then the common ratio of the arithmetic sequence is___ ．

Let three unequal real numbers be A-D, a, a + D, (D ≠ 0) after exchanging the positions of the three numbers: ① if a is the equal ratio median, then A2 = (A-D) (a + D) solution gets d = 0, which does not conform; ② if A-D is the equal ratio median, then (A-D) 2 = a (a + D) solution gets d = 3A, then the three numbers are a, - 2A, 4a, and the common ratio is - 2 or three