Ai ca (n+1)! - n! = n! x n( faci si tu un mic calcul!)
Conform acestei observatii => 2! - 1! = 1! x 1;
3! - 2! = 2! x 2
4! - 3! = 3! x 3
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(n+1)! - n! = n! x n
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Aduni " in cascada" => (n+1)! - 1 = 1! x 1 + 2! x 2 + 3! x 3 +...+ n! x n;
Bafta!