If x ∈ m, then 1 / 1-x ∈ m, then when 4 ∈ m, the product of all elements of M is equal to

If x ∈ m, then 1 / 1-x ∈ m, then when 4 ∈ m, the product of all elements of M is equal to

For 1 / (1-x), replace x with 1 / (1-x), then 1 / (1-1 / (1-x)) = 1 / (- X / (1-x)) = (1-x) / (- x) = 1-1 / X
Then substitute 1 / (1-x) for X, 1-1 / (1 / (1-x)) = 1 - (1-x) = X
So x ∈ m, 1 / (1-x) ∈ m, 1-1 / X ∈ m,
When 4 ∈ m, - 1 / 3 ∈ m, 3 / 4 ∈ m,
So m has elements 4, - 1 / 3,3 / 4,
The product of the elements of set M is 4 * (- 1 / 3) * 3 / 4 = - 1
(actually, X * 1 / (1-x) * (1-1 / x) = - 1)