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13+26+39+...........715


Răspuns :

13 + 26 + 39 + ......... + 715 = 
13(1 + 2 + 3 +............+ 55) =
13 * 55 * 56 : 2 =
13 * 55 * 28 =
20 020
1+2+3+...+n=n*(n+1)/2

13+26+39+...+715=13*(1+2+3+...+55)=13*55*56/2=13*55*28=20020