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Combinari de n+1 luate cate n-1 repede va rog frumos..

Răspuns :

(n+1)! / [ (n-1)! * (n+1 -(n-1)) =(n+1)! /[ (n-1)! *2 ] =n*(n+1)/2 
(n+1)!/(n-1)!*2!=(n-1)!*n*(n+1)/(n-1)!*2=n*(n+1)/2