Question

Rezolvati ecuatia : C n la 2-21=0

Answer 1

$C^2_n - 21 = 0$
$C^2_n = 21$

$C^2_n = \frac{n!}{(n-2)!*2!} = \frac{(n-2)! * n(n-1)}{2(n-2)!} = \frac{n(n+1)}{2}$

$\frac{n(n+1)}{2} = 21$
$n^2 - n - 42 = 0$
Δ = 169 
n1 = 7 ∈ N
n2 = -6 ∉ N