[tex]\displaystyle \\
1+5+9+13+ \hdots +25 = \frac{\text{(numarul de termeni)(ultimul + primul) }}{2} \\ \\
\texttt{Calculam numarul de termeni "n":} \\ \\
n = \frac{\texttt{ultimul termen - primul termen}}{\texttt{diferenta dintre 2 termeni vecini}} +1 = \\ \\
= \frac{25 - 1}{4}+1 = \frac{24}{4}+1 = 6 + 1 = \boxed{\texttt{7 termeni} } \\ \\
\text{Pentru ca sunt putini, ii putem scrie pe toti si ii numaram: } \\
"1+5+9+13+ 17+24 +25" ~~~\text{Numaram si gasim 7 termeni}
[/tex]
[tex]\displaystyle \\
\texttt{Calculam suma:} \\ \\
1+5+9+13+ \hdots +25 = \frac{7(25+1)}{2} = \frac{7 \times 26}{2} = 7 \times 13 = \boxed{91}[/tex]
