[tex]\displaystyle\bf\\Consider~ca~suma~este~asa:\\\\3+7+11+...+83=?\\\\Calculam~numarul~de~termeni\!:\\\\n=\frac{83-3}{4}+1=\frac{80}{4}+1=20+1=21~de~termeni.\\\\Aplicam~formula~lui~Gauss.\\\\S=\frac{Nr.~de~termeni~(~ultimul+primul~)}{2}\\\\S=\frac{n(83+3)}{2}=\frac{21\times 86}{2}=21\times43=\boxed{\bf903}[/tex]