Question

Se considera suma S=1+2+3+...+n. Determinati numarul natural n, daca acesta reprezinta 5% din S.

Answer 1

S=$\frac{n(n+1)}{2}$
$\frac{5}{100} S= \frac{1}{20} \frac{n(n+1)}{2} \\ n=\frac{1}{20} \frac{n(n+1)}{2} \\ 40n=n(n+1) \\ 40=n+1 \\ n=39$