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20+40+60+80+....+1200

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+40+60+80+....+1200 =

= 20(1 + 2 + 3 + ...+ 60)

= 20· 60·61 : 2 

= 36600