[tex]\displaystyle\bf\\1+5+9+13+...+725=?\\\\Calculam~numarul~de~termeni\!:\\\\n=\frac{725-1}{4}+1\\\\n=\frac{724}{4}+1\\\\n=181+1\\\\\boxed{\bf n=182}\\\\Folosim~formula~lui~Gauss\!:\\\\1+5+9+13+...+725=\frac{n(725+1)}{2}=\frac{182\times726}{2}=\\\\=91\times726=\boxed{\bf 66066}[/tex]
