MCQ
જો $_{11}P_r = _{12}P_{r-1} $ હોય, તો $r$ ની કિંમત ...... થાય.
- A$8$
- ✓$9$
- C$12$
- D$13$
$ _{11}P_r = _{12}P_{r-1} $
$\therefore \frac{11!}{(11-r)!} = \frac{12!}{(12-r+1)!}$
$\therefore \frac{(13-r)!}{(11-r)!} = \frac{12!}{11!}$
$\therefore \frac{(13-r)(12-r)(11-r)!}{(11-r)!} = \frac{12×11!}{11!}$
$\therefore (13-r) (12-r) = 4×3 $
$\therefore 13-r = 4 \Rightarrow r=9 $
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