[tex]\displaystyle \bf\\
3+8+13+18+ \cdots +103 = ?\\\\
\text{\bf Calculam numarul termenilor:}\\\\
n = \frac{103 - 3}{5}+1 = \frac{100}{5}+1 = 20+1 = 21~de~termeni\\\\
3+8+13+18+ \cdots +103 = \\\\
= \frac{21(103+3)}{2}= \frac{21\times 106}{2}=21\times 53=\boxed{\bf 1113}[/tex]