n! = 1*2**n
n ∈ N
a>b
Aₐᵇ = a! / (a - b)!
Cₐᵇ = a! / (a - b)!b!
Aₓⁿ⁻³ : Aₓⁿ⁻² = x!/(x - n + 3)! : x!/(x - n + 2)! = x!/(x - n + 3)! * (x - n + 2)!/x! = 1 / (x - n + 3)
(x - n + 3)! = 1*2**(x - n + 2)(x - n + 3)
(x - n + 2)! = 1*2**(x - n + 2)
(x - n + 2)! / (x - n + 3)! = (x - n + 3)
Cₓⁿ⁻³ : Cₓⁿ⁻² = x!/(x - n + 3)!(n - 3)! : x!/(x - n + 2)!(n - 2)! = x!/(x - n + 3)!(n - 3)! * (n -2)!(x - n + 2)!/x! = (n - 2) / (x - n + 3)
(n - 2)! = 1*2**(n - 4)(n - 3)(n - 2)
(n - 3)! = 1*2**(n - 4)(n - 3)
(n - 2)! / (n - 3)! = n - 2
1/( x - n + 3) = 1/8
(n - 2)/(x - n + 3) = 5/8
(n - 2) / 8 = 5/8
n - 2 = 5
n = 7
x - n + 3 = x - 7 + 3 = x - 4 = 8
x = 12
