Pn=n!
Pn-4=(n-4)!
Pn-2=(n-2)!
(n-2)!=(n-4)!*(n-3)*(n-2)
1/(n-4)!=20/(n-2)!
(n-2)!=20*(n-4)!
(n-2)!=(n-4)!*(n-3)*(n-2)=20*(n-4)!
(n-3)(n-2)=20
n^2-5n+6=20
n^2-5n-14=0
n=7 (n=-2 < 0 не подходит)
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Answers & Comments
Pn=n!
Pn-4=(n-4)!
Pn-2=(n-2)!
(n-2)!=(n-4)!*(n-3)*(n-2)
1/(n-4)!=20/(n-2)!
(n-2)!=20*(n-4)!
(n-2)!=(n-4)!*(n-3)*(n-2)=20*(n-4)!
(n-3)(n-2)=20
n^2-5n+6=20
n^2-5n-14=0
n=7 (n=-2 < 0 не подходит)