a) 2+4+6+....+88 =
= 2 × (1 + 2 + 3 +...+ 44) =
= 2 × 44 × (44 + 1) : 2 =
= 2 × 22 × 45 =
= 44 × 45 =
= 1980
b) 2+6+10+14+....+82 =
= 2 × (1 + 3 + 5 + 7 +...+ 41) =
Aflam cati termeni are suma:
(41 - 1) : 2 + 1 = 40:2 + 1 = 21
= 2 × (41 + 1) × 21 : 2 =
= 42 × 21 =
= 882
