Question

Determinați suma elementelor mulțimii S={3,8,13,18,...,103}

Answer 1

   
[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]