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afla sumele 30+60+90+120+...+720

Răspuns :

30(1+2+3+...+24)
30(24*25):2=15*600=9000
Dai pe 30 factor comun:
30(1+2+3+4+...+24)= 30(24•26/2)
30(12•25)=30•300=9000