[tex]\displaystyle\bf\\3+7+11+15+...+43=?\\\\Pasul~1:\\Calculam~numarul~de~termeni~(n).\\\\n=\frac{43-3}{4}+1=\frac{40}{4}+1=10+1=11~termeni.\\\\Pasul~2:\\Aplicam~formula~lui~Gaus.\\\\3+7+11+15+...+43=\frac{n(43+3)}{2}=\frac{11\times46}{2}=11\times23=\boxed{\bf253}[/tex]