1 + 5 + 9 + ...+ 41 =
aflam cati termeni are suma:
(41 - 1) : 4 + 1 = 40:4+1 = 11
= (1 + 41) × 11 : 2 =
= 42 × 11 : 2 =
= 21 × 11 =
= 231
sau
4×0+1 + 4×1+1 + 4×2+1 +...+ 4×10+1 =
= 4× (0 + 1 + 2 +...+ 10) + 1 + 1+...+ 1 de 11 ori
= 4 × 10 × 11 : 2 + 1 × 11 =
= 4 × 5 × 11 + 11 =
= 11 × (20 + 1) =
= 11 × 21 =
= 231
