[tex]\displaystyle
1+3+5+...+(2n-1)=n^2 \\
\texttt{Calculam numarul de termeni din sir: } \\ \\
\frac{(2n-1)-1}{2}+1 = \frac{2n-2}{2}+1 = \frac{2(n-1)}{2}+1 =n-1+1 = n\,termeni \\ \\
1+3+5+...+(2n-1)=n^2 \\ \\
\frac{n[(2n-1)+1]}{2}=n^2 \\ \\
\frac{n[2n-1+1]}{2}=n^2 \\ \\
\frac{n \times 2n}{2}=n^2 \\ \\
n \times n=n^2 \\
cctd[/tex]