Răspuns:
[tex](4*1-1); (4*2-1); (4*3-1); (4*4-1)...\\\\4*20-1=80-1=79-al20lea.termen\\\\\\S = 3+7+11+15+...+79\\\\S= (1+2)+(3+4)+(5+6)+(7+8)+...+(39+40)\\\\S= 1+2+3+...+40\\\\S= 40*(40+1):2= > S= 820-suma.primilor.20.termeni\\\\\\4*51-1=203-al51lea-termen[/tex]
