[tex]\displaystyle\bf\\1+3+5+7+...+99=?\\\\Calculam~numarul~de~termeni\!:\\\\n=\frac{99-1}{2}+1=\frac{98}{2}+1=49+1=\boxed{\bf50~de~termeni}\\\\Calculam~suma~termenilor:\\\\1+3+5+7+...+99=\frac{n(99+1)}{2}\frac{50\cdot100}{2}=50\cdot50=50^2=\boxed{\bf2500}[/tex]
