यदि `""^(12)P_(r) = 1320` तो सिद्ध कीजिए कि `r = 3`

Explanation

`""^(12)P_(r) = 1320 rArr (12!)/((12-r)!) =1320 = 12.11.10= (12.11.10.9!)/(9!) = (12!)/(9!)`
`rArr (12-r)! rArr 12-r = 9`
`rArr r = 3`