permutation and combination
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nP4=12×nP2
nP4/nP2=12
n!/(n-4)!/n!/(n-2)!=12
(n-2)!/(n-4)!=12
(n-2)!/(n-4)(n-3)(n-2)!=12
Now (n-2)! Cancel
So 12(n-4)(n-3)=0
,n=3,4 ans
nP4/nP2=12
n!/(n-4)!/n!/(n-2)!=12
(n-2)!/(n-4)!=12
(n-2)!/(n-4)(n-3)(n-2)!=12
Now (n-2)! Cancel
So 12(n-4)(n-3)=0
,n=3,4 ans
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