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Calculează după model
30+60+90+...........+900
Model:
15+30+45+..........+600=
=15×1+15×2+15×3+.........+15×40
=15×(1+2+3+........+40)
=15×40×41:2
=600×41:2
=24600:2
=12300

VA ROG DAU COROANA!!


Răspuns :

30+60+90+...+900 =

= 30 × 1 + 30 × 2 + 30 × 3 +...+ 30 × 30 =

= 30×(1 + 2 + 3 +...+ 30) =

= 30 × 30 × 31 : 2 =

= 900 × 31 : 2 =

= 27 900 : 2 =

= 13 950