Question

2+4+6+....+numarul ABC=numarul ABC00
Carei multimi apartine nr ABC?

Answer 1

ABC ∈ numerelor naturale pare
notam ABC cu 2n
2(1+2+3+......+n) = 2n·100
2·n(n+1) /2 = 200n
n² + n - 200n = 0
n(n - 199) = 0    n = 199      ABC = 398
2(1+2+3+.....+199) = 199·200 = 39800