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2+3+4+......+30 9+10+11+......+30


Dau coroana


Răspuns :

2+3+4+......+30  = (2+30)·29:2 = 32:2·29 = 16·29 = 464

30-1 = 29 termeni

9+10+11+......+30 = (9+30)·22:2 = 39·11 = 429

30-9+1 = 22 termeni