If `P(11 , r)=P(12 , r-1)` find `ndot`

Explanation

Given,`P (11,r) = P(12,r-1)`


=`(11!)/((11−r)!)=(12!)/([12−(r−1)]!)`

=`(11!)/((11−r)!)=(12×11!)/([12−r+1]!)`

=`(1)/((11−r)!)=(12)/([13−r]!)`

=`(13−r)(12−r)=12`

=`156−13r−12r+r^2=12`

=`r^2−25r+156−12=0`

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