Răspuns :
a) Arătaţi că: 1 + 2 + 3 + … (n- 2) + (n – 1) + n = n(n+1):2 A=1 + 2 + 3
+ … (n - 2) + (n - 1) + n }
│ → A
+ A= (n +1) + [ (n- 1) + 2] +
A = n + (n -
1) + (n - 2) + … + 3 + 2 + 1}
+ … + [ (n-2) + 3] + [(n – 1) + 2] + (n + 1).
Deci 2A= (n +
1) + (n – 1 + 2) + (n – 2 + 3) +… + (n – 2 + 3) + (n – 1 + 2) + (n + 1) →
2A= (n + 1) +
(n + 1) + (n + 1) + …+ (n + 1) + (n + 1) + (n + 1) → 2A= n(n + 1) →
A= 1 + 2 + 3 + … + (n –
2) + (n-1) + n =n(n+1):2