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S=1+2+3+...+2009= ajutatima repede va rog

Răspuns :

Facem suma lui Gauss care consta in formula S=n*(n+1):2, adica
2009*2010:2=40338090:2=2019045
[(2009+1)×2009] :2= (2010×2009):2= 1005×2009= 2019045