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Identificati un factor comun, in fiecare suma , calculati:
10+20+30+...+=1230=
7+14+21+...+315=


Răspuns :

10+20+30+...+1230=10(1+2+3+...123)=10*123*124/2=76260
7+14+21+...+315=7(1+2+3+...+45)=7*45*46/2=7245