下列说法都是错的,试说明错在哪里? 1 设等比数列{an}的公比q,|q|<1,则此数列为单调递减数列。 2 b~2=ac是a、b、c组成等比数列的充要条件 3 设n为奇数a,x_1,x_2…,x_n, b均为实数,且at<0,则a,x_1,x_2,…x_n,b可以组成一个等比数列。 4 已知a,b,c,d成等比数列,则a+b,b+c,c+d成等比数列。 The following arguments are all wrong. Try to explain where it is wrong? 1 Set the common ratio q of the series {an}, q|<1, then the series is monotonically decreasing. 2 b~2=ac is a necessary and sufficient condition for a, b, and c to compose a geometric sequence 3 Let n be an odd number a, x_1, x_2..., x_n, b all be real numbers, and at<0, then a, x_1, x_2 ,...x_n,b can form a geometric sequence. 4 It is known that a, b, c, and d are equal number sequences, and a+b, b+c, and c+d are equal number sequences.