It is known that the sum of the coefficients in the expansion of (3x-1 / X squared to the third power) is 128

It is known that the sum of the coefficients in the expansion of (3x-1 / X squared to the third power) is 128

The sum of the coefficients in the expansion of (3x-1 / X squared to the third power) is the value of "the nth power of" (3x-1 / X squared to the third power) "when x = 1
So the nth power of (3-1) is 128
So: n = 7
In addition, in the 7th power of (3x-1 / X squared cubic), the term 1 / X & sup3; is C [7] (6) &; (3x) [(- 1 / X squared cubic)] = 21 / X & sup3;
Therefore, the coefficient of the third power of 1 / X in the expansion is 21