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De rezolvat in N ecuatia:
(n-1)!/(n-3)!=20


Răspuns :

(n-1)!/(n-3)!=20

(n-1)(n-2)(n-3).....3×2×1/(n-3).....3×2×1=20
(n-1)(n-2)=20 
20=5×4    ⇒ 
n=6

verificare 
(n-1)!/(n-3)!=20
(6-1)!/(6-3)!=5!/3!=(5×4×3×2×1)/(3×2×1)=5×4=20