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Proof by mathematical induction

If 12Pr = 1...

Question

If $^{1}^{2} P_{r} =$ 1320, then r $=$

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Solution

The correct option is B 3
$^{12} P_{r} = 1320$
$\frac{12!}{(12 - r)} = 1320$
$(12 - r)! = \frac{12!}{1320}$
$(12 - r)! = 362880$
$(12 - r)! = 9!$
$r = 3$
Alternatively
$^{12} P_{1} = 12$
$^{12} P_{2} = 132$
$^{12} P_{3} = 1320$
$r = 3$

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