Explicație pas cu pas:
n>=4, altfel combinarile nu au sens
n!/[4!*(n-4)!=5n(n-3)/6
n(n - 1) * (n - 2) * (n - 3) / 1 * 2 * 3 * 4 = 5n * (n - 3) / 6 n(n - 1) * (n - 2) * (n - 3) / 24 = 5n * (n - 3) / 6
n(n - 1) * (n - 2) * (n - 3) / 4 = 5n * (n - 3)
(inmultim 14)
n(n - 1) * (n - 2) * (n - 3) = 20n * (n - 3)
(impartim cu n(n - 3)
(n - 1)(n - 2) = 20 n ^ 2 - 3n + 2 = 20
n ^ 2 - 3n - 18 = o
(n + 3)(n - 6) = 0
n - 6 = 0
n = 6
x4
: n(n - 3)
