Question

Aratati ca x apartine lui N a.i. 2+5+8+..+x=155

Answer 1

(2 +3·0) + (2 + 3·1) + (2 + 3·2) +.........+(2 + 3·n) = 155
2·(n+1) + 3(1 + 2 + 3 +........+n) = 155
2(n+1) + 3·n(n+1)/2 = 155
4n + 4 + 3n² + 3n = 310
3n² + 7n - 306 = 0
Δ = 49 + 3672 = 3721    √Δ = 61    n = (-7 + 61)/6 = 54/6 = 9
x = 2 + 3n = 29