[tex]\displaystyle \text{Aflam numarul de termeni: } \\ \\
n= \frac{99-1}{2}+1 =\frac{98}{2}+1 =49+1=50 \\
Folosim formula: \\
S= \frac{(\text{Numarul de termeni}) \times (\text{Suma dintre primul si ultimul termen})}{2} \\ \\
1+3+5+...+99= \frac{n(99+1)}{2}=\frac{50(100)}{2}=50 \times 50 = \boxed{50^2}=\boxed{2500}[/tex]
