Răspuns :
uite. si tine minte regula de sus de pe primul rand
n(n-1)/2=n+2
n(n-1)=2(n+2)
n²-n=2n+4
n²-n-2n-4=0
n²-3n-4=0
Δ=b²-4ac=3²+16=9+16=25 >0
n₁=(3+5)/2=8/2=4 ∈ N
n₂=(3-5)/2=-2/2=-1 ∉ N
n=4 ∈ N