The absolute values of the coefficients of the second, third and fourth terms of the (1-x) ^ n expansion form an arithmetic sequence. Try to find the middle term of the expansion

The absolute values of the coefficients of the second term, the third term and the fourth term are n, n (n-1) / 2, n (n-1) (n-2) / 6, respectively,
It is known that N + n (n-1) (n-2) / 6 = n (n-1),
Divide both ends by n to get 1 + (n-1) (n-2) / 6 = n-1,
It is reduced to n ^ 2-9n + 14 = 0,
The solution is n = 7 or n = 2 (rounding off)
Expand a total of 8 items, and there are two items in the middle, which are the fourth item T4 = C (7,3) (- x) ^ 3 = - 35x ^ 3,
The fifth term T5 = C (7,4) (- x) ^ 4 = 35x ^ 4

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