Răspuns :
20+40+60+80+....+1200 =
= 20×(1 + 2 + 3 + 4 +...+ 60) =
= 20× 60 × (60+1) : 2 =
= 1200 × 61 : 2 =
= 600 × 61 =
= 36 600
= 20×(1 + 2 + 3 + 4 +...+ 60) =
= 20× 60 × (60+1) : 2 =
= 1200 × 61 : 2 =
= 600 × 61 =
= 36 600