Răspuns:
[tex]S = \frac{n(n + 2)}{4} [/tex]
Explicație pas cu pas:
[tex]S = 2 + 4 + 6 + ... + n \\ = 2 \times (1 + 2 + 3 + ... + \frac{n}{2} ) \\ = 2 \times \frac{( \frac{n}{2})( \frac{n}{2}+1)}{2} = \frac{n}{2}\times \frac{n + 2}{2} \\ = \frac{n(n + 2)}{4} [/tex]
