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Calculati sumele 1+2+3+...+30,,, 2+3+4...30, ,,, 9+10+11+...+30

Răspuns :

Icecon2005

1 + 2 + 3+...+ 30 =[30 × (30+1)]:2 = 465

2 + 3 + 4...30 = (1 + 2 + 3+...+30)-1 = [30 × (30+1)]:2-1 =  465 -1 = 464

9 + 10 + 11 +...+30  = 1 + 2 + 3+...+30  - (1+2+3+.... + 8) =[30 × (30+1)]:2 - [8 × (8+1)]:2 = 465 - 36 = 429

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