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Greu! Cat face 1+2+3+4+5+6+7+8+9+10+..................+98+99
............ inseamna in continuare  +11+12


Răspuns :

1 + 2 + 3 + 4 + ..... + 99 = ( 1 + 99) : 2 * 99 = 100 : 2 * 99 = 50  99 = 4950
99(99+1):2 = (99*100): 2 =9900:2=4950