Please explain the marked step .




 

Dear student
We have,n!9! (n-9)!=n!8! (n-8)!19! (n-9)!=18! (n-8)!19×8!×(n-9)!=18!  (n-8) (n-9)!   n!=n(n-1)(n-2).....3.2.19=n-8n=17
Regards

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