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Rezolvati ecuatia : C n la 2-21=0


Răspuns :

[tex]C^2_n - 21 = 0[/tex]
[tex]C^2_n = 21[/tex]

[tex]C^2_n = \frac{n!}{(n-2)!*2!} = \frac{(n-2)! * n(n-1)}{2(n-2)!} = \frac{n(n+1)}{2} [/tex]

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