Question

combinari de n luate cate 2 = 6

Answer 1

D: n>0

$C _{n} ^{2} =6 \\ \frac{n!}{2!(n-2)!} = 6 \\ \frac{(n-2)!(n-1)n }{2(n-2)!} = 6 \\ (n-1)n = 6*2 \\ n^{2} -n - 12 = 0$

Δ=49 => 
n1 = (1+7)/2 = 4
n2 = (1-7)/2 = -3 ∉ D

=> n = 4

Answer 2

D: n>0



Δ=49 => 
n1 = (1+7)/2 = 4
n2 = (1-7)/2 = -3 ∉ D

=> n = 4